BIOLOGY 

LIBRARY 

G 


APPLIED  BIOLOGY 

AN   ELEMENTARY   TEXTBOOK   AND 
LABORATORY   GUIDE 


BY 

MAURICE   A.    BIGELOW,   PH.D. 

PROFESSOR  OF  BIOLOGY  IN  TEACHERS  COLLEGE,  COLUMBIA 

UNIVERSITY;  CO-AUTHOR  OF  THE  "TEACHING  OF 

BIOLOGY  IN  THE  SECONDARY  SCHOOL" 

AND 

ANNA   N.    BIGELOW,    M.S. 

TEACHER   OF   HIGH-SCHOOL   BIOLOGY 


THE   MACMILLAN   COMPANY 
1911 

-All  rights  reserved, 


ECOLOGY 

LIBRARY 

G 


COPYRIGHT,  1911, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.     Published  October,  1911. 


Nortoooti 

J.  8.  Gushing  Co.  —  Berwick  &  Smith  Co. 
Norwood,  Mass.,  U.S.A. 


PKEFACE 

THIS  book  is  intended  for  use  as  a  combined  textbook  and 
practical  guide  for  a  year's  course  of  five  hours  per  week. 
It  attempts  to  select  from  the  fields  of  botany,  zoology,  and 
human  biology  the  essential  facts  and  especially  the  great 
ideas  of  the  science  of  life  which  are  of  interest  to  the  average 
intelligent  person  who  has  no  time  and  reason  for  more  exten- 
sive study  of  biology. 

The  word  "  applied  "  in  the  title  of  this  volume  should  not 
be  understood  in  the  limited  sense  of  economics,  nor  solely 
with  reference  to  human  physiology  and  hygiene.  It  is  true 
that  in  these  two  lines  biology  has  vastly  important  applica- 
tions to  human  life,  but  it  must  not  be  overlooked  that  in 
certain  phases  the  science  has  value  in  the  intellectual  and 
aesthetic  life  of  cultured  citizens.  Hence,  in  the  most  liberal 
interpretation,  "  applied  biology "  must  present  those  facts 
and  ideas  of  the  science  which  apply  to  human  life  in  its 
combined  intellectual,  esthetic,  economic,  and  hygienic  out- 
look. It  has  been  the  aim  of  the  authors  to  select  for  this 
book  the  facts  and  ideas  in  these  lines  which  seem  best 
adapted  to  that  stage  of  education  which  for  the  vast  majority 
of  students  is  the  climax  of  formal  education.  In  other  words, 
it  has  been  attempted  to  present  the  science  of  biology  applied 
to  the  daily  life  of  the  average  intelligent  citizen. 

This  book  presents  an  order  of  study  and  selection  of  mate- 
rials which  have  long  appealed  to  the  authors  as  a  very 
helpful  answer  to  the  widespread  criticisms  of  the  common 
elementary  courses  of  botany  and  zoology  from  the  pure- 
no  £  r»  f\  f\ 


VI  PREFACE 

science  point  of  view.  It  is  far  from  being  merely  a  theo- 
retical attack  upon  the  problems  of  an  introductory  course 
in  biology,  for  a  large  part  of  the  radically  new  arrangement 
of  subject  matter  has  been  worked  out  in  practice  by  the 
authors  and  by  several  other  teachers  who  have  worked  under 
suggestions  from  the  authors. 

The  authors  trust  that  the  critical  readers  whose  attention 
may  be  attracted  to  parts  of  this  book  which  seem  to  depart 
radically  from  the  traditional  introduction  to  biology,  espe- 
cially by  means  of  separate  courses  in  botany  and  zoology, 
may  be  so  kind  as  to  frame  their  criticisms  in  the  light  of  the 
explanations  and  defense  which  have  been  written  for  an 
accompanying  "  Teachers'  Manual  of  Biology."  See  foot-note 
on  the  first  page  of  Chapter  I. 

A  noticeable  feature  of  the  book  is  omission  or  at  least  lack 
of  emphasis  upon  several  hundred  technical  terms  commonly 
used  in  elementary  books  of  botany  and  zoology.  Italics  have 
been  used  for  emphasizing  the  most  important  scientific  terms 
when  introduced  and  defined,  while  technical  words  in  the 
plain  type  should  be  understood  by  students  as  important 
for  special  study,  for  temporary  use  in  the  text,  or  for  com- 
parison with  other  books.  The  authors  realize  that  in  select- 
ing the  biological  terms  for  emphasis  they  may  have  omitted 
some  really  useful  terms,  and  it  may  be  that  certain  italicized 
words  which  have  a  narrow  range  of  application  do  not  deserve 
the  emphasis  given  them.  A  further  discussion  of  this  ques- 
tion of  technical  words  will  be  included  in  the  "Teachers' 
Manual." 

This  book  also  departs  widely  from  the  traditional  teaching 
of  biological  sciences,  in  that  it  presents  much  laboratory  work 
in  the  form  of  demonstrations  by  the  teacher  instead  of 
entirely  as  individual  work  for  the  students.  This  introduces 
an  important  problem  which  can  be  adequately  presented  only 
in  the  " Teacher s'  Manual";  but  the  experience  of  many 


PREFACE  Vll 

science  teachers  in  secondary  schools  and  colleges  is  leading 
towards  some  golden  mean  between  the  old-time  lecture  and 
demonstration  method  and  the  more  recent  laboratory  work 
for  individual  students.  The  authors  have  suggested  for 
demonstrations  all  practical  work  which  experience  shows  is 
ill-adapted  for  individual  study  by  students  with  limited  time 
and  training.  See  foot-note  on  page  10. 

.  One  of  the  authors  has  discussed  the  meaning  of  the  move- 
ment towards  a  course  in  introductory  biology,  instead  of 
separate  courses  in  botany  and  zoology,  in  Chapter  V  of  Part  II 
in  Lloyd  and  Bigelow's  "  Teaching  of  Biology  in  the  Secondary 
School,"  and  also  in  School  Science  and  Mathematics  for  Octo- 
ber, 1908;  and  the  principles  there  stated  have  been  held 
fundamental  while  making  this  book  of  "Applied  Biology." 
See  also  the  "  Teachers'  Manual." 

While  it  would  be  easily  possible  to  select  from  this  book 
material  for  a  very  elementary  course  of  biology  in  the  first 
year  of  some  high  schools,  the  authors  have  planned  to  issue 
a  smaller  book  especially  arranged  with  reference  to  the 
present  peculiar  conditions  obtaining  in  high  schools  where 
a  course  of  biology  comes  in  the  first  year. 

The  authors  acknowledge  the  useful  suggestions  and  con- 
structive criticisms  which  they  have  received  from  many 
teachers  of  biology ;  and  especially  are  they  grateful  for  the 
help  on  numerous  problems  which  have  been  discussed  with 
Professor  H.  M.  Blchards  and  Professor  C.  C.  Curtis,  of 
Columbia  University,  and  with  Miss  Jean  Broadhurst  and 
Miss  Caroline  Stackpole,  of  the  Department  of  Biology  in 
Teachers  College,  Columbia  University.  Also,  acknowledg- 
ments are  due  publishers  and  authors  who  have  granted  per- 
mission for  the  use  of  numerous  illustrations  from  standard" 
books.  As  far  as  sources  were  known  to  the  authors,  credit 
for  figures  has  been  given  in  the  legends. 

The  authors  cordially  invite  correspondence  from  teachers 


V1U  PREFACE 

who  have  suggestions  arid  criticisms,  or  who  need  help  in 
conducting  courses  along  the  lines  laid  down  in  this  "  Applied 
Biology'7  and  the  accompanying  "Teachers'  Manual  of 
Biology." 

M.    A.    B. 

NEW  YORK  CITY, 
September,  1911. 


CONTENTS 
PAET  I 

INTRODUCTORY   STUDY:    PRINCIPLES   OF  BIOLOGY 

CHAPTER  PA6K 

I.     BIOLOGY:    THE  SCIENCE  OF  LIFE    ......         1 

II.     CHANGES  AND  COMPOSITION  OF  LIFELESS  AND  LIVING  MATTER        5 

III.  THE  CHARACTERISTICS  OF  LIVING  THINGS      .         .  .10 

I.     Chemical  Composition  of  Living  compared  with  Life- 
less Things.     II.  Life-Activities  :  (a)  Animals,  (&)  Plants 

IV.  THE  STRUCTURE  AND  LIFE  OF  AN  ANIMAL  :    INTRODUCTION 

TO  ANIMAL  BIOLOGY 23 

The  Structure  (Anatomy)  of  the  Frog  ....  25 

The  Tissues  of  the  Frog  :  Introduction  to  Microscopic  Study  37 
The  Work  of  Organs  of  the  Frog :  Introduction  to  Animal 

Physiology 44 

Development  of  the  Frog  :  Introduction  to  Embryology  .  57 

Classification  of  the  Frog 64 

V.     THE  STRUCTURE  AND  LIFE  OF  A  PLANT  :   INTRODUCTION  TO 

PLANT  BIOLOGY         ........  66 

The  Structure  of  a  Bean  Plant 67 

The  Reproduction  of  the  Bean  Plant 76 

The  Work  of  the  Organs  of  a  Plant :  Introduction  to  Plant 

Physiology         .........  85 

Classification  of  the  Bean  Plant 121 

VI.     COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY       .         .         .     122 

VII.     CLASSIFICATION  OF  ANIMALS  AND  PLANTS      ....     133 

ix 


CONTENTS 


PART    II 

PRINCIPLES   OF  BIOLOGY   ILLUSTRATED   BY   TYPES 
OF   PLANTS 

CHAPTER  PAGE 

VIII.     STUDIES  OF  SEED-PLANTS      .......  146 

Seeds  and  Seedlings 146 

Roots  of  Seed-Plants 156 

Stems  of  Seed-Plants 163 

Leaves  of  Seed- Plants 188 

Flowers  of  Seed-Plants 196 

Seed-Plants  without  True  Flowers  :  Gymnosperms    .         .  213 

Fruits  of  Seed-Plants .216 

Seed-Plants  Reproducing  without  Flowers          .         .         .  225 

General  Notes  on  Seed-Plants 228 

IX.     STUDIES  OF  SPORE-PLANTS 232 

I.     Higher  Spore-Plants  —  Ferns,  Mosses        .        .         .  233 

II.     Lower  Spore- Plants  —  Algae,  Fungi  ....  245 

III.     Bacteria  276 


PAET    III 

PRINCIPLES   OF  BIOLOGY   ILLUSTRATED  BY   TYPES 
OF  ANIMALS 

X.     THE  SIMPLEST  ANIMALS  :   PROTOZOA    .....  300 
XI.     THE    SIMPLEST    MANY-CELLED    ANIMALS  :     PORIFERA    AND 

CCELENTERATA               ........  320 

XII.     THE  WORM-LIKE  ANIMALS 340 

XIII.  THE  ECHINODERMS 355 

XIV.  THE  ARTHROPODS 35C 

Crustaceans         .........  358 

Arachnids .         .         .376 

Myriapods 379 

Insects 380 

XV.     THE  SHELL-ANIMALS  :   MOLLUSCA         ...                 .  405 


CONTENTS  XI 

CHAPTER  PAGE 

XVI.      THE  VERTEBRATES .         .417 

Fishes 419 

Amphibians .         .         .  424 

Reptiles 426 

Birds 429 

Mammals    ..........  436 

Life-Histories  of  Vertebrates  442 


PART   IV 

PRINCIPLES   OF   BIOLOGY  APPLIED  TO   HUMAN 
STRUCTURE   AND   LIFE 

INTRODUCTION.     HUMAN  BIOLOGY  AND  CLASSIFICATION  OF  MAN     .     466 
XVII.     HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES          .         .         .     467 

XVIII.     BIOLOGY  APPLIED  TO  HEALTHFUL  LIVING          .        .         .     525 
Personal  Hygiene     ........     625 

Effects  of  Stimulants  and  Narcotics         .        .        .        .539 

Bacteriology  applied  to  Human  Health  ....     554 

PART   V 
XIX.     EVOLUTION  AND  HEREDITY  OF  ANIMALS  AND  PLANTS  661 


PART   I 

INTRODUCTION   TO  BIOLOGICAL   STUDY 


CHAPTER   I 
BIOLOGY:   THE   SCIENCE    OF   LIFE 

1.*  Living  and  Lifeless  Things.  —  Since  the  science  of 
biology  deals  with  living  things,  it  is  of  importance  that  at 
the  beginning  of  our  study  we  should  stop  to  consider  that 
all  things  which  we  know  through  our  senses  are  either  living 
or  lifeless.  This  will  be  evident  if  we  attempt  to  write  the 
names  of  some  common  things,  grouping  them  according  to 
whether  they  appear  to  have  life  or  not.  It  is  not  difficult 
to  decide  that  air,  soil,  minerals,  and  water  belong  in  the 
list  of  lifeless  things  and  that  the  most  common  animals  and 
plants  are  examples  of  the  living;  but  we  are  puzzled  by  such 
objects  as  dry  seeds,  undeveloped  eggs  of  animals,  some 
plants  in  the  winter  condition,  and  many  microscopic  ani- 
mals which  show  no  signs  of  life  when  dry.  Are  such  things 
living  or  lifeless  ?  Usually  it  is  not  possible  to  answer  until 
time  and  proper  conditions  have  given  an  opportunity  for 
changes  which  suggest  life-activities.  However,  such  uncer- 
tain cases  must  be  left  undecided  until  after  a  careful  study 
of  the  differences  between  living  and  lifeless  things. 


*  In  the  "Teachers'  Manual  of  Biology"  designed  to  accompany  this 
book  there  will  be  found,  in  paragraphs  numbered  to  correspond  with  those 
in  this  textbook,  notes  on  books,  materials,  and  methods  of  interest  to 
teachers,  or  to  advanced  students  in  normal-school  classes, 

B  1 


-APPLIED*  BIOLOGY 


2.  Organisms,    Organic    and    Inorganic    Matter.  —  Except 
in  science  study,  we  rarely  stop  to  think  of  the  facts  brought 
out  in  the  problem  above;  but  for  the  purposes  of  our  later 
work  in  science  it  is  important  that  we  stop  and  make  such 
a  survey  as  above  suggested,  and  recognize  clearly  that  in 
this  world  of  ours  there  are  two  kinds  of  things,  —  the  living  * 
(collectively   called  animals   and   plants)    and    the   lifeless 
(e.g.,  air,  soil,  water,  minerals,  etc.).     Living  things  are  in 
science  commonly  called  organisms,  and  the  substance  of 
which  their  bodies  are  composed,  or  which  they  form,  is 
organic   matter.     Lifeless   substance   which    has   not   been 
formed  by  organisms  is  called  inorganic  or  mineral  matter. 
All  substances,  then,  in  living  and  lifeless  things  are  composed 
of  matter  which,  as  we  learn  through  our  five  senses,  exists  in 
many  different  forms. 

3.  The  Sciences.  —  Knowledge  regarding  the  living  and 
lifeless  things  of  nature  is  systematically  arranged  in  the 
natural  sciences.     A  common  division  of  these  sciences  is  that 
into  (1)  the  physical  sciences  (chemistry,  physics,  geology, 
mineralogy),  and  (2)  the  biological  sciences,  which  are  con- 
cerned with  living  things.     We  shall  see  later  that  there  is 
much  chemistry  and  physics  used  in  the  study  of  living 
things,  and  hence  it  will  be  made  clear  that  these  two  sciences 
deal  not  only  with  lifeless  things,  but  also  with  all  substances 
and  changes  which  are  found  in  both  living  and  lifeless 
things. 

4.  Biology,    Botany,    and    Zoology.  —  The    study    which 
this  book  will  direct  deals  primarily  with  living  things,  both 
plants  and  animals.     Biology  is  the  science  which  treats  of 


*  Throughout  this  book  italics  are  used  for  words  and  phrases  which  are 
very  important  in  biology,  and  especially  for  scientific  words  where  they 
are  first  introduced  and  defined.  Such  emphasized  words  and  their  mean- 
ings should  be  given  special  attention  by  students.  Technical  biological 
words  which  have  a  very  limited  use,  perhaps  applicable  to  only  a  few 
animals  or  plants,  are  printed  in  plain  type  ;  and  also  they  are  in  parentheses 
if  not  important  except  for  reference  to  other  biological  books. 


BIOLOGY:     THE  SCIENCE  OF  LIFE  3 

life  and  all  living  things  —  plants,  animals,  and  man.  There 
are  two  subdivisions  of  biology;  namely,  zoology,  treating 
of  animals,  and  botany,  treating  of  plants.  Zoology  is  often 
called  animal  biology,  and  botany,  plant  biology.  The  division 
of  biology  into  botany  and  zoology  does  not  mean  that  plants 
and  animals  are  entirely  unlike;  but,  on  the  contrary,  we 
shall  find  later  that  these  two  kinds  of  living  things  have 
many  points  of  remarkable  similarity  in  both  structure  and 
activities.  This  similarity  is  especially  striking  in  many 
microscopic  living  things  which  so  combine  both  plant  and 
animal  characteristics  that  biologists  have  not  decided 
whether  they  are  animals  or  plants.  But  although  there  is 
often  a  great  similarity  between  animals  and  plants,  it  is 
sometimes  convenient  to  study  the  two  kinds  of  living  things 
separately,  and  so  biological  science  is  subdivided  into 
botany  and  zoology.  These  two  subdivisions  of  biology 
are  most  important  for  advanced  students;  but  the  best 
and  most  interesting  beginning  study  is  that  which  directs 
attention  to  the  great  facts  common  to  all  living  things. 
Hence  this  book  for  beginners  is  called  "  Biology,"  to  indicate 
that  it  uses  both  animals  and  plants  to  illustrate  facts  and 
ideas  which  are  true  of  all  living  things. 

5.  Applied  Biology.  —  More  important  than  the  similarity 
of  animals  and  plants  is  the  fact  that  many  animals  are  in 
structure  and  activities  very  much  like  the  human  body; 
and  hence  the  study  of  animals  and  plants  helps  us  to  under- 
stand better  the  human  body  and  its  life-activities.  More- 
over, thousands  of  animals  and  plants  profoundly  affect 
human  life.  For  example,  they  provide  all  the  food-supply 
for  mankind ;  many  harmfully  influence  human  health ; 
and  some,  such  as  pet  animals  and  ornamental  plants, 
contribute  much  to  the  pleasures  of  life.  Clearly,  some 
knowledge  of  the  science  of  living  things  ought  to  be  of 
great  interest  to  educated  citizens,  because  it  applies  in  so 
many  ways  directly  or  indirectly  to  human  life.  Recogniz- 


4  APPLIED  BIOLOGY 

ing  this  fact,  it  is  the  aim  of  this  book  (1)  to  call  attention  to 
the  most  important  facts  and  principles  to  be  learned  by  the 
study  of  selected  animals  and  plants,  and  then  (2)  to  show 
how  biological  science  applies  to  everyday  human  life. 
This  book  is  therefore  entitled  "  Applied  Biology  " ;  that 
is  to  say,  biology,  the  science  of  all  life,  is  applied  to,  or  looked 
at  from  the  standpoint  of,  human  life  as  represented  in  the 
experiences  and  interests  of  intelligent  people  in  general. 


CHAPTER   II 

CHANGES  AND  COMPOSITION  OF  LIFELESS  AND  LIVING 

MATTER* 

IN  our  later  studies  of  living  things  (animals,  plants,  and 
man)  we  shall  often  need  to  have  in  mind  some  important 
facts  and  principles  relating  to  the  composition  of  both 
living  and  lifeless  things  and  to  the  changes  which  occur 
in  them ;  and  these  are  outlined  in  this  chapter. 

6.  Three   States   of   Matter.  —  Soil,   water,   and  air  are 
forms  of  lifeless  matter  which  are  examples  of  the  three 
states,  solid,  liquid,  and  gaseous,  in  which  matter  exists. 

Matter  in  one  of  these  states  may  be  transformed  into 
either  of  the  other  states.  Thus  water,  which  is  ordinarily 
liquid,  may  be  cooled  and  frozen  into  ice  (the  solid  state), 
or  it  may  be  heated  and  changed  into  vapor  or  steam  (the 
gaseous  state).  Iron  and  other  common  metals,  which  are 
ordinarily  solid,  may  be  melted  into  the  liquid  state  and  at  an 
extremely  high  temperature  may  even  change  to  a  gaseous 
state.  Liquid  air  is  made  by  reducing  the  temperature  to 
—  312°  F.  by  subjecting  air  to  great  pressure  in  powerful 
machines. 

7.  Physical  Change.  —  In  all  such  changes  of  matter  from 
one  state  to  another  (from  solid  to  liquid,  or  to  the  gaseous, 
etc.)   the  same  substance  continues  to  exist.     Ice  is  only 
solid  water,  steam  is  a  gaseous  state  of  water,  molten  iron 
cools  into  solid  iron,  and  sugar  and  salt  will  dissolve  in  water. 
In  .these  cases  there  has  been  a  change  in  the  state  of  matter, 


*  Students  who  have  previously  taken  courses  in  chemistry  and  physics 
should  read  this  chapter  as  a  review  of  familiar  facts,  but  from  a  new  view- 
point. 

5 


6  APPLIED   BIOLOGY 

but  not  a  change  in  composition.  Such  changes  of  state 
which  do  not  affect  the  composition  of  substances  are  called 
physical  changes. 

That  branch  of  science  which  treats  of  the  form  and  physi- 
cal changes  of  matter  produced  by  heat,  light,  sound,  elec- 
tricity, gravitation,  etc.,  was  formerly  called  natural  phi- 
losophy, but  is  now  usually  known  as  physics. 

8.  Chemical  Change.  —  All  matter  is  subject  to  another 
kind  of  change  in  which  the  composition  is  affected  and  new 
substances  are  formed.     Ordinary  burning  of  wood  or  gas, 
and  dissolving  baking  soda  in  vinegar  or  other  acid,  are  com- 
mon examples.     The  substances  burned,  or  dissolved  in  acids, 
are    changed    to    other    substances.     Such    transformations 
which  affect  the  composition  of  matter  are  chemical  changes. 

9.  Elements.  —  The  nature  of  chemical  change  will  be 
clear  after  some  further  consideration  of  the  composition 
of  matter.     In  chemistry,  the  science  which  treats  of  the 
composition  and  chemical  changes  of  substances,  we  learn 
that  all  forms  of  matter  —  all  living  and  lifeless  substances 
in  land,  air,  and  water  —  are  composed  of  about  80  elements, 
of  which  about  20  are  very  common.     Some  of  these  ele- 
ments exist  naturally  in  the  solid  state ;   for  example,  iron, 
copper,  lead,  sulphur,  gold,  nickel,  silver,  platinum,  carbon, 
magnesium,   aluminum,   tin,   and  zinc.     Some  others,   like 
mercury  (quicksilver),  are  liquid;   and  still  others  are  gases, 
of  which  the  two  known  as  oxygen  and  nitrogen  constitute 
the  greater  part  of  the  air. 

Chemical  Symbols.  —  For  convenience  in  writing  the  names 
of  the  elements,  chemists  have  adopted  certain  symbols  or 
abbreviations.  The  ones  which  will  be  most  needed  for  ref- 
erence in  this  book  are :  H  for  hydrogen ;  N,  nitrogen ; 
O,  oxygen;  C,  carbon;  S,  sulphur;  P,  phosphorus;  Na, 
sodium  (or  natrium) ;  K,  potassium  (or  kalium) ;  Fe,  iron 
(or  ferrum) ;  Ca,  calcium  (lime).  A  table  giving  the  full  list 
may  be  found  in  any  elementary  textbook  of  chemistry. 


CHANGES  OF  LIFELESS  AND  LIVING  MATTER         7 

10.  Compounds  of  Elements.  —  Now,  the  elements  have 
the  power  of  combining  with  each  other  so  as  to  form  various 
compounds.  To  illustrate :  the  burning  of  magnesium  is 
a  chemical  combination  between  magnesium  and  oxygen  of 
the  air  —  two  elements  are  here  united  to  form  a  new  sub- 
stance, which  is  a  compound  of  magnesium  and  oxygen 
and  is  known  as  oxide  of  magnesium.  Such  a  union  of  any 
substance  with  oxygen  is  called  oxidation.  The  burning 
of  coal  is  in  part  a  combination  between  the  elements  oxygen 
and  carbon,  but  coal  contains  elements  besides  carbon. 
All  ordinary  burning  or  combustion  is  an  oxidation;  that  is, 
the  forming  of  a  combination  between  oxygen  of  the  air  and 
some  other  elements.  In  all  cases  of  chemical  change  there 
is  a  combining  of  elements  into  new  or  different  substances. 
Such  substances  composed  of  two  or  more  elements  are 
called  compounds.  Most  of  the  materials  in  the  solid  matter 
of  the  earth  are  compounds;  water,  which  is  the  most 
abundant  substance,  is  a  compound  of  the  elements  hydrogen 
and  oxygen;  and  most  of  the  materials  in  the  bodies  of 
animals  and  plants  are  compounds.  Air,  however,  is  not  a 
chemical  compound;  the  nitrogen  and  oxygen  are  not 
united  into  a  new  substance,  but  are  simply  mixed  together, 
just  as  dry  sand  and  sugar  can  be  mixed  without  change  of 
composition  in  either.  Other  minor  constituents  of  the 
air  will  be  mentioned  later. 

Using  the  chemical  symbols  for  the  elements,  chemists 
write  MgO  for  magnesium  oxide  formed  by  burning  magne- 
sium (Mg)  in  the  oxygen  of  the  air,  and  ZnO  for  zinc  oxide 
formed  by  burning  zinc  in  the  same  way.  The  formula  MgO 
expresses  the  composition  of  a  molecule  of  magnesium  oxide 
and  means  that  it  is  made  up  of  one  atom  of  magnesium 
united  with  one  of  oxygen.  This  is  the  simplest  possible 
compound.  In  most  compounds  of  two  or  more  elements 
there  is  a  greater  proportion  of,  certain  elements,  and  this 
is  expressed  by  numbers  after  the  elements  of  which  there  is 


8  APPLIED  BIOLOGY 

more  than  one  atom  in  the  compound.  Examples  are  :  water, 
written  H2O,  meaning  that  two  atoms  of  hydrogen  (H)  are 
combined  with  one  .of  oxygen  (O) ;  and  sulphuric  acid, 
written  H2S04,  meaning  that  two  atoms  of  hydrogen,  one 
of  sulphur,  and  four  of  oxygen  are  combined  to  form  the 
acid. 

In  the  above  example  of  burning  magnesium  there  are  two 
elements;  but  more  than  two  elements  are  often  involved 
in  a  chemical  change.  One  compound  of  several  elements 
may  cause  a  chemical  change  in  another  compound,  as  dis- 
solving baking  soda  (HNaCOs)  in  sulphuric  acid  (H2S04) 
illustrates.  This  dissolving  of  baking  soda  in  acid  illustrates 
the  most  common  kind  of  chemical  change,  for  most  sub- 
stances on  the  earth  are  compounds.  It  has  been  noted 
that  the  burning  of  magnesium  involves  only  two  elements, 
but  the  ordinary  burning  of  coal  and  other  common  fuels  is 
a  union  of  compounds  with  oxygen,  resulting  in  several 
new  compounds  in  the  smoke  and  ashes. 

Disintegration  of  Compounds.  —  Not  only  may  elements 
unite  to  form  compounds,  but  these  may  be  separated  into 
simpler  ones  or  even  into  their  constituent  elements.  For 
example,  water  may  be  formed  by  burning  hydrogen  gas 
so  that  oxygen  of  the  air  unites  with  the  hydrogen  as  shown 
by  the  formula  H2O,  but  water  may  also  be  decomposed  by  an 
electric  current  passed  through  it  in  a  suitable  apparatus 
and  the  two  constituent  elements  (hydrogen  and  oxygen) 
in  gaseous  form  be  collected  separately.  Such  disintegra- 
tion of  compounds  into  the  constituent  elements  is  not  so 
common  in  nature  as  is  change  of  compounds  into  simpler 
ones. 

11.  Composition  and  Changes  of  Living  Matter.  —  The 
principles  stated  in  this  chapter  have  been  illustrated  by 
lifeless  matter,  but  we  shall  see  in  later  lessons  that  they 
are  also  applicable  to  living  matter.  Living  things  are 
composed  of  a  number  of  common  elements  united  in  very 


CHANGES   OF  LIFELESS   AND  LIVING   MATTER         9 

complex  substances,  and  in  their  bodies  there  are  constant 
physical  and  chemical  changes  connected  with  all  life-activi- 
ties. Especially  do  foods  eaten  and  oxygen  from  the  breathed 
air  become  involved  in  numerous  changes  in  the  living 
matter  of  animal  and  plant  bodies.  Many  times  in  later 
lessons  we  shall  need  to  refer  to  the  principles  of  chemistry 
and  physics  which  this  chapter  reviews. 

Summary :  (1)  Both  living  and  lifeless  matter  may  un- 
dergo physical  and  chemical  changes;  and  (2)  all  matter  is 
composed  of  elements,  usually  combined  in  compounds,  and 
capable  under  certain  conditions  of  new  recombinations  into 
other  compounds. 


CHAPTER  III 
THE    CHARACTERISTICS    OF   LIVING   THINGS 

I.    CHEMICAL  COMPOSITION  OF    LIVING    COMPARED 
WITH  LIFELESS  THINGS 

IN  order  to  understand  the  relations  of  living  and  life- 
less things,  we  need  to  know  whether  the  organic  matter  of 
living  things  is  composed  of  peculiar  substances  which  are 
not  found  in  inorganic  matter.  A  few  simple  experiments 
will  show  some  remarkable  similarity  of  composition. 

12.  Water  in  Living  Things.  —  Water,  which  is  itself  life- 
less, forms  a  large  part  of  the  bodies  of  animals  and  plants,  as 
the  following  experiments  show. 

(D)*  With  a  delicate  balance,  weigh  carefully  a  piece  of  plant 
stem  or  a  leaf,  record  the  weight,  place  in  a  warm,  dry  place  (e.g., 
over  a  radiator,  lamp,  or  stove,  or  in  sunlight),  and  when  dry  weigh 
again.  The  loss  in  weight  represents  approximately  the  amount  of 


*  Directions  for  practical  work  are  printed  in  the  smaller  type  throughout 
this  book.  Problems  for  individual  work  in  the  laboratory  are  marked  (L). 
Most  of  the  laboratory  work  requires  the  supervision  of  a  teacher,  but  it  will 
be  found  that  most  exercises  require  little  or  no  apparatus  and  may  be  con- 
ducted in  the  ordinary  class-room  if,  as  in  many  schools,  there  is  no  special 
laboratory. 

The  letter  Z>,  at  the  beginning  of  a  paragraph  in  small  type,  indicates  that 
the  practical  work  suggested  is  recommended  for  demonstration  by  the 
teacher ;  but  when  marked  D  or  L  it  may  be  assigned,  if  the  teacher  pre- 
fers, as  a  problem  for  the  individual  work  of  the  students. 

All  subject-matter  in  the  larger  type  is  suitable  for  recitations.  In  most 
cases  the  laboratory  problems  and  the  demonstrations  should  be  taken  in 
regular  order,  because  much  of  the  text  in  larger  type  interprets  and  supple- 
ments the  practical  work.  In  preparing  any  section  for  recitation  or  examina- 
tion, students  should  review  the  laboratory  directions  in  small  type,  for  many 
fundamental  facts  there  given  are  necessary  for  the  intelligent  reading  of 
the  text  in  larger  type. 

10 


THE  CHARACTERISTICS  OF  LIVING  THINGS         11 

water  that  has  evaporated.     Keep  the  dry  piece  of  plant  for  a  later 
experiment. 

(D)  Get  a  small  piece  of  raw  meat,  or  a  frog  killed  by  chloroform. 
Wipe  dry  with  blotting-  or  filter-paper,  and  weigh.  Dry  and  weigh 
again,  as  in  the  case  of  the  plant  in  the  first  experiment.  What 
proportion  of  the  animal  substance  is  water?  Compare  with  the 
plant. 

These  experiments  give  us  a  general  idea  of  the  large 
amount  of  water  in  animal  and  plant  bodies.  It  would 
require  very  careful  experiments  with  very  delicate  apparatus 
for  weighing  and  special  apparatus  for  drying  in  order  to 
determine  the  exact  amount  of  water  in  plants  and  animals. 
Careful  investigations  by  chemists  have  shown  that  the  body 
of  a  higher  animal  (e.g.,  a  dog)  is  nearly  70  per  cent  water. 
This  is  believed  to  be  true  also  of  the  human  body.  This 
water  is  derived  from  that  which  we  drink  and  also  from  foods. 
Thus  potatoes  contain  about  78  per  cent  water,  milk  85  per 
cent,  tomatoes  over  90  per  cent,  apples  over  80  per  cent,  and 
lean  meat  over  50  per  cent. 

It  is  evident  that  water  must  play  an  important  part  in 
the  life  of  animals  and  plants,  and  in  this  connection  it  is 
interesting  to  note  the  abundance  of  water  and  its  wide 
distribution  over  the  earth.  All  good  modern  textbooks  of 
geography  emphasize  the  close  relation  between  the  distribu- 
tion of  water  and  that  of  animal,  plant,  and  human  life. 

13.  Gaseous  substances,  which  are  themselves  lifeless,  may 
be  obtained  from  the  bodies  of  animals  and  plants. 

(D)  Place  the  plant  material  left  after  drying  in  the  above  ex- 
periment in  the  bowl  of  a  clay  tobacco-pipe,  close  the  mouth  of  the 
bowl  with  soft  clay  or  plaster  of  Paris,  support  the  pipe  by  a 
wire  or  by  a  retort-stand,  heat  the  bowl  in  the  flame  of  a  gas-  or 
alcohol-burner  until  it  reddens  and  smoke  (gases)  begins  to  issue 
from  the  pipestem,  then  light  the  smoke  with  a  match.  In  an- 
other pipe  heat  some  splinters  of  wood  or  sawdust,  and  burn  the 
gas  in  the  same  way.  A  piece  of  dried  meat  heated  in  the  same 
way  gives  off  gases,  which  may  be  burned. 


12  APPLIED  BIOLOGY 

Such  experiments  prove  that  combustible  gases  may  be 
obtained  from  animal  and  plant  substances  after  the  water 
has  been  removed.  Chemists  have  shown  that  these  gases 
are  not  present  as  such  in  the  animals  and  plants,  but  the 
elements  of  which  they  are  composed  are  united  in  chemical 
compounds  which  change  to  gas  when  heated.  This  will  be 
clear  when  we  recall  that  coal,  wood,  or  oil  (all  of  which  are 
of  organic  origin)  when  heated  in  the  retorts  at  the  gas-factory, 
give  off  illuminating  gas.  Gas  as  such  is  not  present  in  the 
solid  coal,  but  the  coal  has  compounds  whose  elements  re- 
combine  when  heated  and  form  the  gas.  So  it  is  in  the  bodies 
of  animals  and  plants;  the  elements  to  form  the  gases  are 
present,  and  the  gases  are  produced  by  heating,  which  causes 
new  recombinations  of  elements. 

14.  Carbon  is  a  prominent  part  of  the  solid  substance  of 
animals  and  plants.     After  the  gases  were  given  off  in  the 
preceding  experiments  a  black   substance   was   left  in  the 
pipe.     The  same  thing  occurs  when  we  burn  an  ordinary 
match;  that  is,  certain  gases  burn  and  produce  the  flame 
and  a  charred  mass  is  left.     This  black  material  we  know  as 
charcoal;  at  the  gas-factory  a  similar  substance  from  heated 
coal  is  called  coke.     The  black  color  of  coke  and  charcoal 
is  due  to  the  presence  of  an  element  known  to  chemists  as 
carbon.     The  easiest  way  to  get  pure  carbon  is  by  charring 
white  sugar  with  intense  heat.     Chemists  know  more  than 
one  hundred   thousand   compounds   of   carbon   with   other 
elements.     It  is  especially  abundant  in  the  organic  com- 
pounds of  which  the  bodies  of  animals  and  plants  are  com- 
posed,  and  consequently  must  be  also  in  the  food  from 
which  the  body  substances  are  made. 

15.  Mineral   substances   enter    into   the   composition   of 
animals  and  plants.     If  we  reheat  the  charcoal  obtained  from 
either  the  animal  or  the  plant  matter  in  the  preceding  demon- 
stration, by  holding  it  with  a  wire  in  the  hot  flame  of  a  gas- 
or  alcohol-burner,  the  carbon  which  it  contains  will  soon 


THE  CHARACTERISTICS  OF  LIVING   THINGS         13 

burn;  that  is,  combine  with  oxygen  to  form  carbon  dioxide 
(C02)}  and  only  mineral  matter  or  ashes  remain.  Another 
example  is  the  familiar  fact  that  when  wood  (or  any  plant 
matter)  is  burned  there  is  formed  a  mass  of  red-hot  coals 
or  embers,  which  if  allowed  to  cool  quickly  become  charcoal ; 
but  if  the  coals  remain  heated,  they  soon  change  to  ashes, 
because  the  carbon  is  burned.  Obviously,  the  charcoal 
obtained  by  heating  ordinary  animal  or  plant  substances 
is  a  mixture  of  combustible  carbon  and  incombustible 
mineral  matter  or  ashes.  Charcoal  made  by  heating  sugar 
leaves  no  ashes,  because  it  .is  pure  carbon,  which  in  burning 
combines  with  oxygen  and  forms  carbon  dioxide. 

A  chemist  could  prove  by  careful  analysis*  that  the  ashes 
from  either  plant  or  animal  substances  are  a  mixture  of 
several  compounds  containing  the  elements  calcium,  sulphur, 
iron,  potassium,  sodium,  and  several  other  elements.  These 
are  all  found  in  the  soils,  and  in  the  water  of  the  lakes,  rivers, 
and  the  sea. 

16.  Summarizing   our    inquiry   into   the   composition   of 
living  things,  we  have  found  that  water,  carbon,  gases,  and 
mineral   matters   may   be   obtained   by   analysis   of   living 
things.     All  these  substances  are  also  found  in  lifeless  or 
inorganic  matter.     It  is  evident  then  that  chemical  composi- 
tion alone  does  not  enable  us  to  distinguish  between  living  and 
lifeless  matter.     And  yet  in  most  cases  we  have  no  difficulty 
in  deciding  whether  a  certain  thing  is  dead  or  alive.     How  do 
we  know?    The  answer  will  be  found  in  the  life-activities 
discussed  in  the  next  lesson. 

II.   LIFE-ACTIVITIES 

17.  Distinguishing    between    Living    and    Lifeless.  —  The 

conclusion  reached  in  the  preceding  lesson  was  that  all  the 
materials  entering  into  the  composition  of  living  things 
(animals  and  plants)  are  also  found  in  inorganic  things, 


APPLIED  BIOLOGY 

such  as  soil,  water,  and  air ;  and  hence  animals  and  plants 
cannot  be  distinguished  from  lifeless  things  by  the  substances 
entering  into  their  composition.  What,  then,  does  distin- 
guish the  living  from  the  lifeless  ?  Let  us  first  try  to  answer 
this  question  by  examining  a  living  animal  (e.g.,  a  frog) 
and,  if  possible,  determine  just  how  the  living  frog  is  different 
in  behavior  from  lifeless  objects,  such  as  a  stone  or  a  dead 
frog. 

LIFE-ACTIVITIES  OF  AN  ANIMAL 

18.  The  living  frog  moves  automatically  or  spontaneously. 
By  this  we  mean  that  within  the  animal  there  is  machinery  for 
producing   motion.      A   stone   or    other   lifeless   thing   can 
move  only  through   the   action   of  some   external   agency ; 
it  may  fall  (gravitation),  or  be  moved  by  swiftly  flowing 
water,  by  a  violent  wind,  or  by  some  animal.     Not  only 
does  the  frog  have  the  power  of  locomotion,  but  also  there 
are  internal  movements,  such  as  breathing  and  the  beating  of 
the  heart,  which  go  on  continuously  as  long  as  the  frog  lives. 

The  statement  that  the  living  frog  has  machinery  for 
producing  motion  reminds  us  of  the  steam-engine,  but  a 
little  study  shows  that  the  engine  is  not  automatic,  as  is 
the  body  of  a  living  animal.  For  example,  the  engine  re- 
quires the  attendance  of  an  engineer  to  supply  it  with  water 
and  fuel  (that  is,  to  feed  it) ;  but  the  frog  is  able  to  obtain 
its  own  food  (fuel)  and  water  to  feed  itself.  Careful  study 
of  all  machines  invented  by  man  shows  that  the  movement 
which  at  first  may  appear  to  be  as  automatic  as  that  of  an 
animal  is  really  dependent  upon  regular  human  attendance. 
Among  common  objects  there  is  nothing  lifeless  which  seems 
to  have  automatic  movement,  and  in  the  vast  majority  of 
cases  it  is  easy  to  decide  whether  an  animal  is  dead  or  alive 
by  simply  testing  for  evidences  of  movement. 

19.  The  living  frog  requires  food  if  it  is  to  continue  to 
live  and  move  and  grow.     This  is  a  fact  so  well  known  that 


THE  CHARACTERISTICS   OF  LIVING   THINGS        15 

it  needs  no  demonstration.  There  is  nothing  like  animal 
growth  among  lifeless  things,  which  increase  in  size  only  by 
addition  of  particles  of  substances  like  themselves.  If  a 
saturated  solution  of  common  alum  (or  copper  sulphate)  be 
made  by  dissolving  as  much  as  possible  in  boiling  water, 
and  then  a  stone  be  dropped  into  the  alum  solution,  it 
will  be  coated  by  alum  crystals,  but  the  mass  of  stone 
will  not  increase.  If  a  lump  of  alum  be  dropped  into 
the  saturated  solution,  the  mass  will  increase  by  the 
addition  of  more  alum  crystals  to  the  surface.  Such  an 
experiment  with  alum,  showing  that  a  mass  of  it  can  increase 
in  size  only  by  addition  of  the  same  substance,  illustrates  a 
fact  applicable  to  stones,  crystals,  minerals,  and  other  in- 
organic things;  they  cannot  add  other  substances  to  them- 
selves and  make  them  really  a  part  of  their  own  bodies. 
On  the  other  hand,  living  animals  can  live  and  grow  on  food 
derived  from  other  animals  or  from  plants.  For  example,  a 
frog  may  eat  smaller  frogs ;  but  it  may  also  eat  earth- 
worms or  plants,  and  the  substance  of  these  will  be  changed 
and  built  into  that  of  the  frog.  This  power  of  the  living 
animal  to  take  food  unlike  itself  and  to  make  it  over  into 
its  own  substance  is  known  as  assimilation  (meaning  to  make 
like  or  identical). 

20.  The  living  frog  breathes,  and  the  lifeless  object  does 
not.  If  we  watch  a  frog,  or  a  higher  animal,  we  see  muscular 
movements  concerned  with  pumping  air  into  and  out  of 
the  lungs;  and  we  call  this  process  breathing.  In  many 
simple  animals  there  are  no  lungs,  but  there  are  several 
ways  of  proving  that  they  require  air  (only  the  oxygen, 
and  not  the  nitrogen  of  the  air),  and  that  they  change  the 
air  when  they  breathe  it.  That  animals  require  air  is  shown 
by  the  fact  that  land  animals  die  very  quickly  if  placed 
in  a  jar  from  which  the  air  has  been  pumped  out  with  an  air- 
pump,  and  aquatic  animals  will  soon  die  in  water  from 
which  the  air  has  been  withdrawn  by  an  air-pump  or  been 


16  APPLIED  BIOLOGY 

expelled  by  prolonged  boiling.  The  following  experiment 
will  show  one  simple  method  of  testing  the  changes  produced 
in  air  which  has  been  breathed :  — 

(D)  Pour  some  lime-water  or  barium-water  into  a  small  bottle 
and  blow  air  from  the  human  lungs  through  a  small  glass  tube  (or 
straw)  into  the  water.  What  happens?  The  change  in  the  lime- 
water  is  due  to  a  gas,  called  carbon  dioxide,  which  is  added  to  the 
air  while  it  is  in  the  lungs. 

The  same  gas  is  formed  by  ordinary  burning.  Wrap  a  piece 
of  wire  around  a  small  candle,  light  it,  and  then  lower  it  into  a  tall, 
wide-mouthed  bottle.  After  a  time  the  flame  will  flicker  out. 
Then  lift  out  the  candle  and  put  in  a  little  lime-water.  Compare 
the  lime-water  with  that  changed  by  air  from  human  lungs. 

Take  a  tall,  wide-mouthed  bottle  or  fruit-jar,  wash  and  rinse 
with  fresh  water,  then  lower  into  the  jar  a  frog  inclosed  in  a  loose  bag 
of  cheesecloth  or  mosquito-netting  with  a  string  attached  so  that 
the  frog  may  be  lifted  out  quickly  without  inverting  the  jar  or  leav- 
ing it  uncovered  more  than  for  a  moment.  One  of  the  preceding 
experiments  suggests  that  human  breathing  changes  the  air  of  rooms ; 
and  hence  it  is  important  that  the  jar  be  held  at  an  open  window  where 
fresh  air  may  enter  while  the  frog  is  being  placed  in  the  jar.  A  good 
plan  is  to  keep  the  jar  full  of  water  until  the  moment  when  ready  to 
place  the  frog  in  it.  The  water  will  prevent  the  jar  becoming 
filled  with  the  air  of  the  schoolroom.  A  second  jar,  treated  exactly 
like  the  first,  but  without  a  frog,  should  be  kept  beside  the  first  for 
comparison.  Leave  the  frog  in  the  jar,  carefully  covered,  for  two 
hours,  quickly  lift  it  out,  pour  in  10  cc.  of  lime-water,  replace  cover, 
and  shake.  Has  any  change  occurred  in  the  lime-water  when  it 
came  into  contact  with  the  air  which  the  frog  had  been  breathing  ? 
Test  the  air  in  the  jar  without  a  frog. 

The  above  experiments  prove  that  some  change  occurs 
in  air  when  animals  breathe  it.  Until  a  later  lesson  we 
need  not  take  time  to  consider  just  what  this  change  is. 
For  our  present  purpose  it  is  sufficient  to  have  demonstrated 
that  animals  do  change  air  when  they  breathe  it  and  that 
lime-water  makes  it  possible  to  demonstrate  breathing  of 
animals  in  which  we  cannot  see  breathing  movements. 

21.  The  living  frog  has  the  power  of  reproducing  new 
animals  like  itself.  Frogs'  eggs  gradually  develop  into  new 


THE  CHARACTEBISTICS   OF  LIVING   THINGS        17 

frogs.  We  shall  later  study  the  development  of  some  animals 
from  eggs,  but  for  our  present  lesson  it  is  sufficient  to  note 
that  books  on  embryology  (the  science  of  development)  state 
that  all  higher  animals  develop  from  eggs,  which  are  small 
masses  of  living  matter  separated  from  the  parent  animals. 
No  lifeless  object  has  any  such  power  of  separating  from  it- 
self a  small  body  which  is  able  to  take  food  and  grow  into 
a  body  like  the  original  one.  The  power  of  reproduction 
is,  then,  a  striking  characteristic  of  living  animals. 

22.  The  frog  after  a  time  loses  the  power  of  moving, 
breathing,  etc. ;    that  is,  it  dies.     We  are  familiar  with  the 
fact  that  animals  of  a  given  kind  or  species  live  for  a  certain 
length  of  time  and  then  grow  old  and  die.     For  example, 
an  elephant  has  been  said  to  live  200  years,  a  horse  40, 
lion  35,  cat  40,  toad  40,  sea-anemone  50,  crayfish  20,  vulture 
118,  eagle  100,  pike  and  carp  200,  squirrel  and  mouse  6, 
pig  20,   sheep  15,  fox  14,  and  hare  10  years.     However,  it 
is  certain  that  most  individuals  of  these  species  live  a  much 
shorter  life ;  for  example,  few  horses  live  over  20  years.     The 
life   of   all   individual   animals,    even   though   they   escape 
accidents  and   disease,   is   of  limited  duration.     They  are 
like  machines,  able  to  run  a  certain  period  of  time.     Ulti- 
mately the  machinery  of  life  stops  and  the  animal  bodies 
rapidly  decompose  into  substances  which  show  no  signs  of 
ever  having  been  living. 

23.  Summarizing,  we  have  seen  that  a  living  animal  has 
the  following  activities :    it  moves ;    it  breathes ;    it  takes 
food   for   assimilation;    it   reproduces.     It   has   still   other 
peculiar  powers  which  will  be  considered  later.     None  of 
these  is  found  in  lifeless  objects,  such  as  stones  or  dead 
animals.     All  such  processes  —  moving,  eating,  breathing, 
reproducing  —  which    are    peculiar    to    living    animals,  are 
known  as  life-activities.     These  are  not  permanent  in  any 
individual  animal,  for  after  a  certain  length  of  life  the  in- 
dividual animal  loses  its  life-activities  —  we  commonly  say 


18  APPLIED  BIOLOGY 

that  it  dies  —  and  the  lifeless  body  soon  changes  to  the  con- 
dition of  inorganic  substances  which  make  up  air,  water,  and 
the  soil  of  the  earth. 

LIFE-ACTIVITIES  OF  A  PLANT 

To  one  who  has  never  studied  botany  it  may  seem  that 
most  of  the  activities  named  above  as  characteristic  of  living 
animals  are  absent  from  living  plants;  but  a  careful  ex- 
amination shows  that  living  plants  move,  breathe,  require 
food,  and  reproduce,  and  in  still  other  ways  resemble  animals 
in  their  life-activities. 

24.  Living  plants  have  movement.  It  is  true  that  most 
plants  with  which  we  are  familiar  are  not  capable  of  locomo- 
tion (i.e.,  movement  from  place  to  place) ;  but  the  same  is 
true  of  many  lower  animals.  On  the  other  hand,  there  are 
many  lower  plants  (to  be  studied  later)  which  have  locomo- 
tion like  that  of  some  lower  animals. 

Locomotion  in  animals  is  only  one  phase  of  their  move- 
ments, and  much  more  impressive  are  the  constant  movements 

of  internal  organs,  such  as  the 
heart  and  the  lungs.  There  are 
many  similar  cases  of  plants  able 
to  move  certain  organs,  e.g.,  the 
Mimosa  ("  sensitive  plant  ") 
moves  its  leaves  and  branches 

FIG.  1.    The   "sensitive  plant"      when  touched   (Fig.  1);  the  Ox- 

(Mimosa).   a,  expanded  leaf;     alis,  the  bean  (Fig.  30),  and  cer- 
being  touched.     tain  ciovers  foid  their  leaflets  at 

night;  the  Venus  fly-trap  (Fig. 
2)  has  peculiar  leaves  able  to  snap  together  and  catch  in- 
sects ;  many  plants  twine  their  stems  around  supports ;  and 
plants  bend  toward  the  light  when  growing  near  a  window. 
All  such  cases  show  that  animals  have  no  monopoly  of  move- 
ment ;  for  in  addition  to  cases  of  plants  which  have  locomotion 


THE  CHARACTERISTICS   OF  LIVING   THINGS         19 

and  others  which  move  leaves  and  other  parts,  there  are  in 
plants  many  movements  which  can  be  detected  only  with  the 
aid  of  a  good  microscope. 

(D)  Examine  any  plants  available  which  show  any  of  the  move- 
ments mentioned  above.  Many  will  be  found  at  greenhouses.  Leaf- 
lets of  the  Elodea  (an  aquatic  plant)  are  excellent  for  the  movements 
visible  with  the  microscope.  A  piece  of  an  oyster's  gill  will  show 
microscopic  movement  going  on  in  animals  (see  §  338). 

25.  Plants  require  food'ii  they  are  to 
continue  to  live  and  grow.     It  is  a  well- 
known  fact  that  ordinary  garden  plants 
will   not  grow  well    unless   there  is  a 
supply  of  fertilizer  (one  kind  of  crude 
plant  food)  in  the  soil.     It  will  be  shown, 

in  a  later  lesson,  that  green  plants  do    Fl_G-  2-   Lea'  °[ 

'  &        •      j      i  fly-trap    adapted    for 

not  grow  well  if  kept  long  in  darkness,       catching  insects, 
because  light  enables  them  to  make  use       (From  Strasburger.) 
of  certain  food  materials. 

26.  Plants    breathe.     In  no  plant  is  it  possible    to   see 
breathing  movements,  as  in  animals;    but  it  is  possible  to 
prove  by  the  lime-water  test  that  changes  are  produced  in 
the  air  by  breathing  of  plants. 

(D)  Take  about  twenty  pea  or  corn  seedlings  (directions  for 
raising  such  seedlings  are  given  in  first  lesson  on  germination  of 
bean,  §  81).  Place  in  a  loose  bag  of  netting,  such  as  was  used  for  the 
experiment  with  the  frog  (§  20).  Then  lower  the  seedlings  into 
a  wide-mouthed  jar  filled  with  fresh  air,  and  after  24  to  48  hours  lift 
out  the  seedlings  and  test  the  air  by  pouring  in  some  lime-water. 
A  second  jar  without  seedlings,  but  otherwise  treated  in  exactly  the 
same  way,  should  be  kept  for  comparison  (control  experiment). 
Take  all  the  care  suggested  for  the  corresponding  experiment  with 
the  frog.  Compare  results  with  that  experiment.  The  breathing 
in  the  plants  is  so  much  slower  than  in  animals  that  the  same  amount 
of  change  in  the  lime-water  is  not  to  be  expected. 

A  potted  plant,  such  as  a  geranium  or  a  begonia,  if  placed  under  a 
bell-jar  beside  a  small  dish  filled  with  lime-water,  will  give  the  same 
proof  that  the  plant  causes  changes  in  air,  just  as  animals  do  when 
breathing. 


20  APPLIED  BIOLOGY 

In  trying  experiments  with  seedlings  or  with  any  full-grown  plants 
which  have  green  color,  keep  the  jar  covered  so  as  to  exclude  light. 
The  reason  for  this  precaution  will  be  clear  in  a  later  lesson  which 
deals  with  the  effect  of  exposing  plants  with  the  green  color  to  light. 

Other  experiments  connected  with  later  lessons  will  give 
further  proof  that  plants  breathe,  and  will  help  to  explain 
how  they  breathe. 

27.  Plants    have    the    power    of    reproduction.     We    are 
familiar  with  the  reproduction  of  many  common  plants  from 
seeds  and  of  others  from  cuttings  or  slips  (e.g.,  many  house 
plants,  such  as  geranium,  coleus,  begonia).     Many  plants 
(ferns,    mosses,    horsetails,    mushrooms,    etc.)    form    small 
bodies,    known   as   spores,    from   which   new   plants   grow. 
And  many  of  the  microscopic  plants  reproduce  by  automati- 
cally dividing  their  bodies  into  two  equal  parts,  and  these 
half-size  plants  soon  grow  to  the  full  size. 

28.  As  in  the  case  of  animals,  individual  plants  ultimately 
die,  but  some  plants  may  live  to  a  very  great  age.     Some  of 
the  giant  Sequoias  of  California,  the  largest  of  which  are 
nearly  300  feet  high  and  more  than  75  feet  in  circumference, 
are  probably  at  least  two  thousand  years  old,  and  some 
botanists  estimate  over  four  thousand  years.     Some  of  the 
famous  oaks  of  Europe  are  believed  to  be  eighteen  hundred 
years  old,  but  our  largest  American  oaks  are  probably  less 
than  five  hundred.     There  are  in  Europe  specimens  of  chest- 
nut, olive,  cypress,  yew,  and  other  trees  which  are  probably 
much  more  than  one  thousand  years  old.     It  is  impossible 
to  estimate  accurately  even  after  the  trees  are  cut  down  and 
the  so-called  "  annual  "  rings  of  the  trunk  counted ;    but 
some  of  these  trees  were  famous  four  or  five  hundred  years 
ago,  and  we  may  be  sure  that  they  are  of  far  greater  age 
than  any  animals  are  known  to  have  reached. 

29.  Characteristic  Life-Activities  of  Animals  and  Plants.  — 
Our  brief  studies  of  a  living  animal  and  a  plant  have  shown 
us  the  following  striking  points  of  resemblance ;  (1)  the  power 


THE  CHARACTERISTICS   OF  LIVING   THINGS         21 

of  movement,  (%}  need  of  food.  (3)  powejjQjLgmffiiwg, 
frnm 


of  rej3n)du£ing.  All  these  activities  or  changes  are  found  in 
living  plants  and  animals  and  none  of  them  in  lifeless  bodies. 
These  are  life-activities  characteristic  of  living  things. 

We  see,  then,  that  in  order  to  distinguish  accurately 
between  living  and  lifeless  things  we  must  determine  whether 
there  are  evidences  of  life-activities.  Every  year  dealers 
in  seeds,  gardeners,  farmers,  and  the  scientists  at  the  govern- 
ment laboratories  must  test  samples  of  seeds  in  order  to 
determine  whether  they  are  living  or  lifeless.  Chemical 
analysis  will  not  settle  such  a  question;  and  so  the  only 
way  to  test  seeds  is  to  put  them  under  conditions  where  some 
or  all  the  life-activities  may  be  manifested.  In  short,  the 
seeds  must  be  planted  under  conditions  favorable  for  growth. 
Likewise  dormant  animals  must  be  carefully  examined  for 
evidences  of  life.  For  example,  small  animals  of  certain 
species  which  are  often  abundant  in  soil  where  pools  of 
water  have  dried  up  in  midsummer  may  appear  perfectly 
dead  when  viewed  with  the  microscope  ;  but  they  begin  to 
move,  eat,  and  manifest  other  life-activities  soon  after  they 
are  placed  in  water. 

30.  The    Machinery   of  Life-Activities.  —  We   have   seer* 
that  certain  activities  in  animals  and  plants  make  the  living 
things  different  from  the  lifeless;  and  we  shall  now  inquire 
concerning  the  stucture  and  working  of  the  living  machinery 
which  in  the  animal  or  the  plant  moves,  takes  food,  breathes, 
grows,  and  reproduces.     But    in  order  to  understand  the 
working  of  any  complicated  machinery,  we  must  first  take  it 
to  pieces  and  examine  its  structure,  and  later  find  out  the 
use  and  work  of  each  part.     To  this  end  we  shall  now  examine 
with  considerable  care  the  structure  of  an  animal,  and  later 
that  of  a  plant. 

31.  Subdivisions  of  Biology.  —  The  science  of  the  structure 
of   animals   and   plants   is   called    anatomy  or  morphology. 


22  APPLIED  BIOLOGY 

The  science  of  minute  structure  as  studied  with  the  aid  of 
the  microscope  is  called  microscopical  anatomy  or  histology. 
The  science  of  life-activities  or  functions  of  living  things  is 
called  physiology;  and  there  are  special  books  devoted  to 
the  sciences  of  plant  physiology,  animal  physiology,  and 
human  physiology.  The  sciences  named  in  the  foregoing 
are  simply  subdivisions  or  departments  of  the  great  science 
of  living  things,  biology. 


CHAPTER  IV 

STRUCTURE  AND  LIFE  OF  AN  ANIMAL  (FROG)  :    AN 
INTRODUCTION    TO    ANIMAL   BIOLOGY* 

32.  Why  the  Frog  is  selected  for  Study.  —  Other  ani- 
mals might  serve  as  types  for  this  study;  but  it  is  better 
to  select  a  backboned  or  vertebrate  animal,  because  such 
an  animal,  by  reason  of  its  great  similarity  to  human  structure 
and  functions,  will  make  it  easier  to  apply  this  introductory 
study  to  later  lessons  on  human  biology.  And  among  the 
backboned  animals  none  has  been  so  popular  for  scientific 
study  as  the  common  frog.  Many  books,  some  of  them 
very  large,  have  been  written  about  the  biology  of  this  animal, 
and  the  scientific  knowledge  concerning  it  is  greater  than 
that  on  any  other  animal.  Strange  as  it  may  seem,  we  know 
far  less  concerning  the  human  body  from  direct  study; 
but  fortunately  the  frog  and  the  human  are  so  much  alike 
in  numerous  ways  that  biologists  have  applied  to  the  human 
species  many  facts  which  were  learned  first  by  study  of  the 
frog.  Since  the  study  of  the  frog  by  scientific  men  has  played 
so  important  a  part  in  building  up  the  science  of  animal 
]biology}  teachers  now  regard  this  animal  as  valuable  for 
study  by  those  who  wish  to  rediscover  for  themselves  some 
of  the  most  important  facts  concerning  animal  structure  and 
life. 

The  common  green  frog  is  usually  most  available  for 
-study,  but  the  descriptions  which  follow  will  fit  any  other 
species  of  frog,  or  even  the  common  garden  toad. 


*  To  THE  TEACHER  :   This  chapter  may  be  studied  after  the  lessons  on  the 
plant,  Chapter  V.    See  note  in  Chapter  IV  in  "  Teachers'  Manual." 

23 


24  APPLIED  BIOLOGY 

33.  Nature-Study  of  Frog  and   Toad.  —  Readers   of  this 
book  who  have  not  studied,  perhaps  in  elementary-school 
nature-study,  the  habits  of  life  of  the  common  toad  and  frogs 
should  read  at  least  one  of  the  following :    "  Usefulness  of 
the  American  Toad,"   in  Farmers'  Bulletin  No.   196   (free 
from  U.  S.  Dept.  of  Agriculture) ;    "  Life  History  of  the 
Toad,"  in  Cornell  Nature-Study  Leaflets;    or  Chapter  16 
in  Hodge's  "  Nature-Study  and  Life." 

34.  Justifiable  Use  of  Animals  for  Science  Study.  —  Some 
people  think  it  wrong  to  kill  frogs  or  other  animals  for  scien- 
tific study,  but  such  persons  do  not  seem  to  have  considered 
points  1  to  6  as  follows:    (1)  Much  useful  knowledge  can 
be  obtained  from   such  study.     (2)  It  is  no  more  wrong 
to  kill  a  few  frogs  painlessly  with  chloroform  than  to  kill 
cattle,  sheep,  and  pigs  for  our  use  as  food.     It  is  not  absolutely 
necessary  that  we  should  have  meat  for  food ;  for  thousands 
of  people   living   healthy   lives   use   only   plant   materials, 
milk,  butter,  eggs,  and  similar  foods.     This  is  not  to  be  taken 
as  meaning  that  animals  should  not  be  used  for  human  food  ; 
that  is  entirely  another  question  which  must  be  answered 
by  the  tastes  of  individuals.     But  one  who  favors  killing 
animals  for  food  purposes  cannot  sincerely  oppose  scientific 
study  of  animals,  for  the  scientific  use  is  no  less  necessary  than 
the  use  as  food.     (3)  Killing  a  few  animals  painlessly  for 
scientific  study  does  not  tend  to  make  the  student  and  teacher 
cruel  and  hard-hearted.     On  the  contrary,  people  who  have 
studied  zoology  extensively  are  usually  very  kind  to  animals 
and  would  not  brutally  kill  a  toad  with  a  stone  or  step  on  an 
earthworm.     (4)  So  few  animals  are  required  for  scientific 
study  that  it  does  not  tend  to  exterminate  any  species.     (5) 
It  is  certainly  far  more  justifiable  to  kill  a  few  animals  pain- 
lessly for  the  purpose  of  scientific  study  than  to  kill  or  in- 
directly to  hire  others  to  kill  animals  for  the  sake  of  articles 
of   decoration,    such    as   bird   plumage   for   millinery.     No 
sensible  person  who  wears  aigrettes  (heron  plumes)  or  seal 


AN  INTRODUCTION  TO   ANIMAL  BIOLOGY          25 

skins  and  who  has  read  the  revolting  but  true  accounts  of  the 
barbaric  cruelty  practised  by  the  hunters  of  the  animals 
which  produce  these  articles  of  decoration  would  ever  question 
the  painless  killing  of  a  few  animals  for  the  sake  of  scientific 
knowledge  which,  as  we  shall  see,  affects  human  life  in  many 
ways.  (6)  No  one  who  approves  of  that  relic  of  barbarism, 
hunting  animals  merely  for  sport  and  trophies,  can  conscien- 
tiously raise  opposition  to  the  scientific  study  of  animals. 

Facts  such  as  the  above  must  lead  us  to  take  a  sensible 
view  of  the  study  of  animals.  Of  course,  the  dissection*  or 
the  taking  to  pieces  of  an  animal  is  not  as  pleasant  work  as 
pulling  a  rose  into  pieces,  but  since  we  want  to  see  for 
ourselves  the  most  important  parts  of  the  machinery  in- 
side an  animal,  we  will  be  sensible,  and  set  ourselves  the 
task  of  carefully  examining  first  the  outside  (external  struc- 
ture) and  then  the  inside  (internal  structure)  of  the  frog. 

THE   STRUCTURE    (ANATOMY)    OF  THE  FROG 

"  The  first  step  towards  an  appreciation  of  animal  life  must  be  taken 
by  the  student  himself,  for  no  booklore  can  take  the  place  of  actual  observa- 
tion. The  student  must  wash  the  quartz  and  dig  for  the  diamonds,  though  a 
book  may  help  him  find  these,  and  thereafter  to  fashion  them  into  a  treasure." 
From  "Study  of  Animal  Life,"  by  Professor  J.  Arthur  Thomson,  of  the  Uni- 
versity of  Aberdeen,  Scotland. 

35.  External  Structure  of  the  Frog.  —  (L)  Place  a  living  frog  in 
a  plain  glass  tumbler,  and  cover  with  paper  or  mosquito-netting. 
The  ordinary  "jelly-glasses"  with  tin  covers  are  convenient,  if 


*  Dissection  is  the  term  commonly  applied  to  the  work  of  separating  a 
dead  animal  or  plant  into  its  organs  in  order  to  learn  the  plan  of  structure. 
Careless  people  sometimes  confuse  dissection  with  vivisection.  The  latter 
term  means  operating  on  living  animals  which  have  been  rendered  insensible 
to  pain  by  means  of  ether,  chloroform,  or  other  anaesthetics.  In  short,  as 
now  practiced  by  the  greatest  investigators,  vivisection  of  animals  is  exactly 
the  same  as  the  surgical  operations  on  human  beings.  Such  operations  on 
animals  are  occasionally  performed  with  the  hope  of  improving  the  methods 
for  surgical  work.  In  fact,  most  of  the  great  operations  which  surgeons  per- 
form would  be  unknown  had  not  operations  on  anaesthetized  animals  shown 
the  proper  methods  for  operating  on  the  human  body. 


26  APPLIED  BIOLOGY 

small  holes  are  punched  in  the  cover.  By  looking  through  the  glass, 
it  will  be  possible  to  learn  many  things  about  the  frog's  external 
structure  and  habits. 

Notice  that  the  frog's  body  consists  of  head,  trunk,  and  limbs. 
How  many  limbs  ?  Is  there  a  neck,  or  a  tail  ? 

Just  as  in  geography  we  use  the  terms  north,  south,  east, 
and  west  to  indicate  directions,  so  in  zoology  we  must  have 
terms  for  directions  or  positions  on  bodies  of  animals.  The 
head-end  of  an  animal  is  called  anterior,  the  opposite  end 
of  the  trunk  is  posterior,  the  lower  surface  of  the  body  is 
ventral,  and  the  back  or  upper  surface  is  dorsal. 

Imagine  your  own  body  supported  on  all  four  limbs  (i.e.,  walking 
on  hands  and  feet)  and  locate  anterior,  posterior,  dorsal,  and  ventral. 
Make  an  outline  sketch  of  the  frog  as  seen  in  profile  and  another  one 
of  a  boy  as  you  imagine  him  walking  on  hands  and  knees,  and  then 
write  the  four  terms  given  above  on  your  sketches  so  as  to  indicate  the 
parts  of  the  body  to  which  they  are  applied.  Compare  the  right 
and  left  sides  of  the  frog.  Remember  that  in  the  study  of  animal 
biology  right  and  left  refer  to  the  frog's  body,  not  to  your  own.  Hold 
the  tumbler  so  that  the  frog  will  sit  with  its  head  pointed  away  from 
you,  and  your  right  will  be  the  frog's  right.  If  the  animal  were 
lying  on  its  back  with  head  pointed  away  from  you,  as  you  look  down 
upon  its  ventral  surface,  would  your  right  be  right  or  left  of  the  frog  ? 
Are  the  two  sides  of  the  frog's  body  alike  ?  Are  the  right  and  left 
sides  of  your  own  body  similar  externally  ? 

Any  animal  having  differentiated  (meaning  made  different) 
dorsal  and  ventral  surfaces,  and  anterior  and  posterior 
ends,  might  be  divided  into  similar  halves  (right  and  left) 
only  by  cutting  in  the  median  plane  from  anterior  to  posterior 
and  from  dorsal  to  ventral.  Can  you  think  of  any  other 
plane  in  which  a  frog  can  be  equally  divided?  Such  an 
animal  is  bilaterally  symmetrical.  Do  you  know  any  animal 
which  is  not  so?  Look  at  a  jellyfish  in  the  school-museum 
or  at  a  picture  of  one  in  a  book  of  zoology.  In  how  many 
places  could  a  wheel  with  eight  spokes  be  divided?  Make 
a  rough  sketch  to  prove  your  answer.  Animals,  like  jelly- 
fishes,  which  have  the  wheel-like  plan  of  structure  are  said 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          27 

to   have   radial   symmetry.     There   are    comparatively   few 
such  animals,  but  numerous  plants  are  so  arranged. 

The  word  organ  will  be  used  frequently  hereafter.  An 
organ  is  a  part  of  the  body  of  a  plant  or  animal  fitted 
for  doing  a  particular  work.  For  example,  the  heart  is  an 
organ  for  pumping  blood,  lungs  are  organs  for  breathing, 
muscles  are  organs  for  moving.  A  group  of  organs  doing 
similar  work  form  a  system.  Thus  all  the  muscles  constitute 
the  muscular  system,  which  is  a  group  of  organs  adapted  to 
the  work  of  producing  movement. 

At  this  point  it  will  be  well  to  have  at  hand  a  dead  frog,  one 
either  recently  chloroformed  or  preserved  for  some  time  in  formalin. 
Examine  the  dead  frog  for  all  points  not  easily  seen  in  the  living 
one  kept  in  the  glass  tumbler. 

Skin.  —  Note  the  color  of  the  skin  on  the  dorsal  surface  of  the 
frog's  body.  Compare  with  the  color  on  the  ventral  surface.  In  a 
later  chapter  it  will  be  pointed  out  that  many  zoologists  think  that 
these  colors  help  to  conceal  the  frog  in  the  grass  along  ponds,  among 
water  weeds,  and  in  other  places  where  frogs  live.  When  you  make 
a  trip  into  the  country  look  for  evidence  that  the  color  helps  to  con- 
ceal the  frog.  The  skin  is  covered  with  a  slime  or  mucus.  How 
would  this  help  the  animal  if  an  enemy  tried  to  catch  it  ? 

Are  there  hairs  on  the  frog's  skin?  Look  with  a  hand-lens. 
Compare  with  your  own  skin. 

Sense-organs.  —  (a)  Examine  eyes.  Are  there  eyelids  ?  Can 
the  frog  close  its  eyes  ?  Look  into  a  small  mirror  and  compare  your 
own  eyes  with  those  of  a  frog.  (6)  Ears  —  the  large,  dark  round 
spots  just  behind  the  eyes  are  the  membranes  of  the  ears,  stretched 
like  a  drum-head  over  the  cavities  (internal  ears),  which  will  be 
examined  later.  The  frog  has  no  projecting  external  ears  such  as 
the  human  being  has.  Also  in  the  human  ear  the  ear-drum  (tym- 
panum) cannot  be  seen  from  the  outside,  because  it  lies  deep  in  a 
canal  or  tube  which  leads  to  the  internal  ear. 

Limbs.  —  Compare  the  anterior  pair  of  limbs  with  the  posterior 
ones.  Are  they  alike  ?  In  the  anterior  limbs  (fore  legs)  notice  three 
divisions :  upper  arm,  extending  from  shoulder  to  elbow ;  fore- 
arm, extending  from  elbow  to  wrist ;  and  the  "hand."  How  many 
"fingers"  ?  Compare  the  anterior  limb  with  your  own;  i.e.,  your 
arm.  What  differences  in  the  divisions  do  you  notice  ?  What 
similarities?  In  the  posterior  leg  (hind  limb  or  hind  leg)  of  the  frog 


28  APPLIED  BIOLOGY 

notice  :  the  thick  fleshy  thigh,  extending  from  the  hip  to  the  knee ; 
the  shank,  extending  from  knee  to  ankle ;  and  the  foot.  How  many 
toes?  Compare  with  number  of  "fingers."  The  shortest  toe 
corresponds  to  the  big  toe  of  the  human  foot. 

Notice  the  membrane  ("web")  stretched  between  the  toes  of  the 
hind  foot,  fitting  the  foot  as  a  paddle  for  swimming.  When  you 
have  an  opportunity  to  observe  frogs  swimming  in  a  large  aquarium, 
or  in  the  clear  water  of  a  pond,  compare  the  uses  of  fore  and  hind 
limbs.  The  fitting  of  any  structure  or  organ  of  an  animal  or  plant 
to  a  special  function  is  known  as  adaptation.  Another  adaptation 

of  the  frog's  hind  legs  is  in  the 
great  muscles  of  the  thigh, 
which  fit  it  for  jumping. 

Mouth.  —  The  opening  is 
the  mouth,  and  inside  is  the 
mouth-cavity  (also  called  buc- 
cal  cavity,  meaning  cheek 
cavity).  Commonly  we  speak 

t*          *~^x  tlM^sfi^^^^    ^~±±L~-    °^  *ke  numan  mouth-cavity 

"        " "  as  mouth,   and  the  opening 

as  lips ;    but  to  be  accurate 
FIG   3.    Various  positions  taken  by  tongue    «mouth»   should  be  applied 
of  frog  when  catchmg  an  insect.     (From          h    Opening.     Examine  the 
Holmes,  after  Wiederscheim.) 

mouth-cavity  of  a  frog  which 

has  been  chloroformed.  Are  there  teeth?  Look  for  teeth  on  a 
mounted  skeleton  of  a  frog.  Notice  two  small  openings  (nostrils)  on 
the  dorsal  side  of  the  head  near  the  upper  lip  of  the  frog's  mouth. 
Pass  a  shoemaker's  bristle  or  a  slender  broom-straw  into  a  nostril,  and 
note  where  the  bristle  comes  out  into  the  mouth-cavity.  The  human 
nostrils  have  no  such  direct  communication  with  the  mouth-cavity. 
With  a  needle  make  a  hole  in  the  frog's  ear-membrane  back  of  the 
eyes,  and  then  carefully  push  a  bristle  into  the  opening.  Open  the 
mouth  and  find  where  the  bristle  comes  out  into  the  mouth-cavity. 

To  prove  a  similar  connection  between  the  human  ear  and  the 
back  part  of  the  mouth-cavity,  close  your  nostrils  with  your  hand, 
then  swallow  once  or  twice  and  notice  a  feeling  of  pressure  in  your 
ears,  due  to  forcing  air  back  into  the  internal  part  of  the  ears.  This 
explains  why  workmen  are  told  to  swallow  air  when  entering  the 
compressed-air  chambers  used  in  tunneling  under  rivers.  The  tubes 
connecting  the  internal  parts  of  the  ears  with  the  mouth  are  known 
as  Eustachian  tubes,  named  in  honor  of  Eustachius,  professor  of 
anatomy  at  Rome,  who  wrote  a  book  describing  the  ear  in  1754. 


AN  INTRODUCTION   TO  ANIMAL  BIOLOGY          29 

Examine  the  frog's  tongue,  and  note  that  it  is  attached  at  the 
forward  end  to  the  tip  of  the  lower  jaw,  while  the  free  end  extends 
backward  towards  the  throat.  How  does  this  compare  with  the 
human  tongue?  In  seizing  an  insect,  the  frog's  tongue  is  turned 
quickly  forward  out  of  the  mouth,  and  then  quickly  withdrawn. 
Figure  3  will  make  clear  the  positions  of  the  tongue  at  various 
steps  of  this  peculiar  movement. 

36.  Internal  Structure  of  the  Frog.  —  (L  when  not  marked  D).  Look 
at  the  mounted  skeleton  of  a  frog,  compare  with  Fig.  4,  and  note 
the  position  of  the  following  bones :  backbone  (vertebral  column) ; 
skull ;  shoulder-girdle,  which  is  a  set  of  bones  attaching  the  fore 
legs  to  the  body ;  pelvis,  a  set  of  bones  which  attach  the  hind  legs 
to  the  body.  Notice  that  the  "ribs"  are  very  short.  Now  turn 
to  the  frog  you  have  been  studying,  and  feel  the  position  of  the 
above-named  bones  through  the  skin. 

Lay  the  frog  on  its  back,  head  pointing  away  from  you.  With 
forceps  lift  the  skin  and  with  scissors  carefully  cut  through  it  along 
the  median  ventral  line  the  whole  length  of  the  body.  Carefully 
separate  the  skin  from  the  underlying  parts,  cutting  the  thread-like 
connections,  turn  the  flaps  of  skin  outward  to  right  and  left,  and 
pin  to  the  board  or  wax  in  the  bottom  of  a  dissecting-pan.  Cover  the 
frog  with  water. 

Notice  the  muscles  of  the  body-wall  of  the  abdomen,  and  the 
bones  connecting  the  fore  limbs  (shoulder-girdle). 

Again  using  scissors  and  forceps,  carefully  cut  through  the  body- 
wall  in  the  median  ventral  line  from  the  pelvis  to  the  shoulder-girdle, 
and  then  cut  across  the  body  (transversely)  just  posterior  to  the 
girdle.  Separate  and  spread  out  the  two  flaps  of  the  body-wall, 
and  pin  down  to  the  dissecting-board.  The  cavity  containing  the 
internal  organs  thus  opened  is  the  body-cavity  (ccelome).  Now  iden- 
tify the  organs  exposed  —  liver,  stomach,  intestine,  egg-organs  or 
ovaries  (if  the  specimen  is  a  female  frog),  comparing  your  specimen 
with  Fig.  5  in  order  to  identify  the  organs. 

Now  cut  out,  with  strong  scissors,  the  ventral  bones  of  the 
shoulder-girdle  (the  teacher  will  demonstrate  how  this  is  best  done). 
As  you  lift  up  the  bones,  notice  the  heart  lying  beneath.  The  poste- 
rior, conical,  whitish  part  of  the  frog's  heart  is  called  ventricle  (Fig. 
5)  ;  it  lies  in  a  depression  between  two  parts  of  the  liver.  Anterior 
to  the  ventricle  are  the  thin-walled  auricles  (right  and  left),  usually 
found  filled  with  dark  blood  in  a  dead  frog.  The  ventricle  is  the  part 
of  the  heart  which  forces  the  blood  from  the  heart  into  the  blood- 
tubes  (blood-vessels) ;  while  the  auricles  are  reservoirs  for  holding 


30 


APPLIED  BIOLOGY 


blood  coming  back  to  the  heart  and  collecting  for  the  next  "beat" 
of  the  ventricle.  The  extremely  thin  membrane  which  incloses  the 
heart  is  the  pericardium  (meaning,  around  the  heart). 


FIG.  4.  Skeleton  of  frog  viewed  from  dorsal.  Nine  segments  (vertebrae) 
and  the  urostyle  (u)  in  backbone,  d,  shoulder-blade  or  scapula,  dorsal  part 
of  shoulder-girdle  ;  h,  humerus,  upper-arm  bone ;  r,  one  of  two  bones  in 
forearm  ;  i,  pelvis  ;  /,  femur  or  thigh-bone  ;  t,  shank-bone.  (From  Mar- 
shall.) 

(D  or  L)  At  the  anterior  end  of  the  heart  is  a  blood-tube  (aorta) 
which  branches  into  two  smaller  tubes,  each  of  which  has  several 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          31 

smaller  branches  leading  to  the  right  and  left  *  sides  of  the  body. 
These  are  the  chief  blood-tubes  through  which  blood  flows  away 
from  the  heart,  and  they  are  called  arteries.  The  branches  of  the 
two  arteries  lead  to  all  parts  of  the  body,  and  may  be  seen  in  many 


FIG.  5.    Frog  dissected  from  ventral  surface,    h,  heart;  Ig,  lungs;   I,  liver; 
m,  stomach ;  d,  intestine  ;   e,  ovaries  ;  6,  bladder.     (From  *Ecker.) 

organs.  Connecting  with  the  anterior  end  of  the  heart  and  on  its 
dorsal  surface  are  three  thin-walled  tubes,  which,  in  dead  frogs,  are 
usually  filled  with  dark-purple  blood.  These  are  the  blood-tubes 
in  which  blood  flows  back  to  the  heart,  and  they  are  called  veins. 


*  The  student  should  remember  that  with  the  frog  lying  on  its  back  the 
right  side  of  the  frog  will  be  on  the  observer's  left.  In  all  descriptions  in  this 
book,  right  and  left  refer  to  the  animal  studied,  not  to  the  observer. 


32 


APPLIED  BIOLOGY 


Later  we  shall  see  small  branches  of  the  three  veins  in  various  parts 
of  the  body.  In  fact  every  organ  which  has  an  artery  in  which 
blood  comes  from  the  heart  must  have  a  vein  for  the  return  of 
blood  to P the  heart;  for  the  blood  circulates  from  heart  to  arteries, 
from  arteries  to  smaller  tubes  called  capillaries,  from  these  to  veins, 


FIG.  6.  Diagram  of  chief  arteries  of 
frog,  p,  to  lungs  ;  c,  to  skin  ;  b,  to 
arm  ;  I,  to  head  ;  m,  to  digestive 
organs  ;  a,  aorta  ;  r,  to  kidneys  ; 
i,  to  legs.  (From  Thomson,  after 
Ecker.) 


Fitf.  7.  Diagram  of  chief  veins  of 
frog,  j,  from  head  ;  b,  from  arm  ; 
c,  from  skin  ;  p,  from  lungs  ;  h,  from 
liver  and  digestive  organs  (i);  k, 
from  kidneys  ;  /,  s,  from  legs.  (From 
Thomson,  after  Ecker.) 


and  thence  back  to  the  heart.  See  diagrams  of  the  circulation  in 
Figs.  6  and  7. 

(D)  Capillaries.  —  Watch  the  flowing  of  blood  through  the 
capillaries  in  the  tail  of  a  small  tadpole  allowed  to  lie  on  its  side  on 
a  wet  plate  of  glass,  or  in  the  spread  web  of  a  frog's  foot.  Use  low 
power  of  microscope. 

In  addition  to  the  system  of  blood-tubes  carrying  blood  from  the 
heart,  through  the  capillaries  in  all  organs,  and  back  again  to  the 
heart,  there  are  H  all  organs  many  small  tubes  which  collect  a 
watery  fluid  called  lymph.  This  fluid  is  derived  from  the  liquid 
part  of  the  blood,  and  it  ultimately  flows  back  into  the  blood. 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          33 

The  largest  organ  in  the  frog's  body  is  the  liver,  a  large  bi-lobed 
(two  lobes  or  divisions)  organ  behind  and  at  the  sides  of  the  heart. 
Its  color  is  reddish  brown  in  frogs  not  preserved  in  chemicals.  It 
will  be  studied  more  carefully  later. 

Ovaries.  —  In  a  full-grown  female  frog,  masses  of  spherical  black 
and  white  eggs  (ova)  lie  among  the  other  organs  (Fig.  5).  These 
ova  are  attached  to,  or  are  really  united  to  form,  two  organs  (the 
ovaries  or  egg-organs)  which  later  will  be  found  to  be  fastened  to  the 
dorsal  wall  of  the  body-cavity,  one  on  the  right  and  one  on  the  left 
of  the  median  line;  i.e.,  bilaterally  symmetrical.  With  forceps, 
pull  out  the  masses  of  ova,  taking  care  not  to  injure  other  structures. 

Alimentary  Organs.  —  These  are  the  organs  concerned  with 
receiving  food  and  preparing  it  for  the  use  of  the  frog's  body.  The 
alimentary  canal,  or  food-tube,  extends  through  the  body  from  the 
anterior  to  the  posterior  extremity  (Fig.  8).  The  mouth  is  the  an- 
terior opening;  the  posterior  opening  is  called  anus.  Turning 
the  organs  in  the  body-cavity,  but  not  cutting,  examine  the 
various  parts  —  stomach,  intestine,  etc.  —  of  the  food- tube.  The 
stomach  lies  dorsal  to  the  left  lobe  of  the  liver.  The  short  tube 
from  stomach  to  throat  is  the  esophagus  (gullet).  The  throat  or 
pharynx  connects  the  esophagus  with  the  mouth-cavity.  Care- 
fully push  a  probe  (such  as  a  small  stick)  down  the  throat  into 
the  stomach.  The  tube  extending  from  the  stomach  backwards  or 
posteriorly  is  the  intestine.  Note  that  the  first  part  lies  parallel 
to  the  stomach.  The  constriction  between  the  stomach  and  this 
first  part  of  the  intestine  is  the  pylorus.  The  small  intestine  is  a 
slender  and  much  convoluted  tube.  The  large  intestine  (or  rectum) 
is  a  short,  straight  tube,  of  greater  diameter  than  the  small  intestine. 
The  expanded  end  of  the  large  intestine  is  called  cloaca.  Tubes  from 
the  kidneys*  and  reproductive  organs  open  into  the  cloaca ;  but  this 
is  not  so  in  the  highest  animals.  The  opening  (anus)  of  the  large 
intestine  to  the  exterior  is  on  the  dorsal  surface  of  the  body  near  the 
end  of  the  backbone. 

Carefully  cut  the  membrane  (mesentery)  which  holds  the  alimen- 
tary canal  in  place,  cutting  close  along  the  canal,  and  pin  the  intes- 
tine to  one  side  and  out  of  the  body-cavity.  Cut  open  the  stomach 
longitudinally.  It  may  contain  food,  such  as  softened  and  disin- 
tegrated pieces  of  worms,  etc. ;  the  condition  of  the  food  suggests 
that  solid  foods  are  dissolved  in  the  stomach.  Wash  out  the  con- 
tents, and  notice  the  longitudinal  folds  which  line  the  stomach  and 
increase  the  surface  with  which  food  comes  into  contact.  Cut  open 
the  intestine  in  several  places.  Do  you  find  folds  arranged  as  in  the 


34 


APPLIED  BIOLOGY 


stomach  ?  Notice  that  the  food  in  the  intestine  is  more  liquid  than 
the  food  in  the  stomach.  Also  notice  that  by  the  time  food  has 
passed  along  the  posterior  part  of  the  intestine  its  bulk  has  been 
greatly  reduced  —  the  stomach's  capacity  is  much  greater  than  that 
of  the  intestine.  Evidently  the  greater  part  of  the  food  disappears 
as  it  passes  along  the  stomach  and  mtes- 
tine.  Where  does  it  go  ? 

In  addition  to  the  parts  of  the  food- 
tube,  the  liver  and  the  pancreas  must  be 
considered  alimentary  organs,  because 
they  form  or  secrete  substances  needed 
for  preparing  foods  in  the  intestine. 

The  liver,  whose  position  has  been  al- 
ready noticed,  consists  of  two  lobes,  the 
lobe  on  the  left  side  of  the  body  being  the 
larger  and  subdivided  into  two  parts. 
Notice  a  small  sac,  the  gall-bladder,  be- 
tween the  right  and  left  lobes.  The 
greenish-colored  fluid  which  fills  the  gall- 
bladder is  the  bile,  which  is  manufactured 
by  the  liver  and  stored  in  the  gall-bladder 
until  it  is  needed  in  the  intestine.  A  bile- 
duct  (very  small  and  difficult  to  see)  leads 
from  the  gall-bladder  to  the  intestine. 

(D)  The  pancreas  lies  between  the 
stomach  and  intestine.  It  is  an  irregularly 
lobed  mass  of  light  color  in  fresh  speci- 
mens. It  secretes  a  fluid  (pancreatic  juice) 
used  in  digestion  of  foods.  Many  of  its 

FIG.  8.    Alimentary  canal  of  sma11  ducts  °Pen  into  the  bile-duct,  which 
frog.     <e,   esophagus ;   m,  extends  along  the  pancreas  on  the  way 
stomach ;  du,  d,  small  in-  from  the  gall-bladder  to  the  intestine, 
testine  ;  py,  pylorus ;  mz,       Cut  the  attachment  of  the  liver,  esoph- 

fc&^blad'de1^6  Z^d*!^  '  agUS'  and  intestine  and  remoye  these  or- 
(From  ^Ecker  )    °  '  *"  £ans'  t^lus  exposing  other  organs  nearer 

the  dorsal  part  of  the  body-cavity. 

(D)  The  lungs  are  two  thin-walled  sacs,  which,  before  removal  of 
the  liver,  were  dorsal  to  that  organ.  Cut  through  the  articulation  of 
the  jaws  so  as  to  allow  the  mouth  to  open  widely,  and  demonstrate 
the  slit-like  glottis  in  the  pharynx  just  ventral  to  the  opening  of  the 
esophagus.  The  tube  into  which  the  glottis  opens  is  the  windpipe 
or  trachea,  which  has  two  branches  leading  to  the  lungs.  Insert  the 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          35 

end  of  a  small  tube  (blow-pipe)  into  the  glottis  and  blow  in  air  to 
inflate  the  lungs ;  or,  using  a  rubber-bulbed  pipette,  fill  them  with 
water. 

(L)  Watch  a  living  frog  in  a  glass  jar,  and  observe  how  the  floor 
of  the  mouth  ("throat")  moves  up  and  down  at  regular  intervals. 
Count  the  movements  in  one  minute.  The  frog  cannot  breathe  as  we 
do  and  practically  "swallows"  air,  forcing  it  down  into  the  lungs  by 
moving  the  floor  of  the  mouth  up  while  the  mouth  and  nostrils  are 
closed.  Many  persons  breathe  while  keeping  the  mouth  open, 
but  a  frog  cannot  breathe  when  the  mouth  is  held  open.  Not  every 
movement  of  the  throat  forces  air  down  into  the  lungs;  most  of 
the  movements  simply  pump  air  into  and  out  of  the  mouth-cavity 
through  the  nostrils.  But  watch  carefully,  and  occasionally  the 
sides  (flanks)  of  the  body  back  of  the  shoulders  will  be  seen  to  expand 
greatly  when  the  frog  seems  to  take  a  big  "swallow"  of  air;  this 
expansion  indicates  that  air  has  been  forced  down  into  the  lungs. 
Then,  after  a  time,  the  muscular  walls  of  the  body  help  the  elastic 
lungs  expel  the  air,  forcing  it  up  into  the  mouth-cavity,  where  it  is 
gradually  mixed  with  fresh  air  pumped  in  through  the  nostrils. 

(D)  The  kidneys  are  flat  bodies  of  a  dark-red  color.  They  are 
attached  to  the  dorsal  body-wall  in  the  posterior  portion  of  the  body- 
cavity,  one  on  each  side  of  the  backbone.  The  ureters  are  the  ducts 
or  tubes  of  the  kidneys  which  lead  to  the  terminal  part  of  the  large 
intestine  (cloaca) ;  they  are  very  small  and  difficult  to  demonstrate. 
The  kidneys  extract  water  and  certain  other  substances  from  the 
blood  which  flows  through  them,  and  then  pass  these  substances  to 
the  exterior  through  the  ureters  and  cloaca.  Before  these  sub- 
stances from  the  kidneys  are  eliminated,  they  may  be  stored  for  a 
time  in  a  sac  known  as  the  bladder,  which  lies  on  the  ventral  side  of 
the  cloaca,  and  opens  into  it.  This  structure  is  usually  found  as  a 
collapsed  membrane  if  one  looks  carefully  before  cutting  out  the 
large  intestine.  Examine  a  museum  specimen  prepared  to  show  the 
bladder  expanded. 

The  spleen  is  a  small,  oval,  red  body  attached  to  the  mesentery 
near  the  large  intestine.  Its  use  will  be  explained  after  we  have 
studied  the  blood  and  lymph. 

Reproductive  Organs.  —  (D  or  L)  Examine  the  rgans  in  specimens 
of  each  sex.  In  the  male  frog  the  spermaries  are  a  pair  of  yellow 
ovoid  bodies  attached  ventrally  to  the  kidneys.  Their  ducts  lead 
into  the  kidneys,  and  there  connect  with  little  tubes  leading  to  the 
ureters,  and  thence  through  the  cloaca  to  the  exterior. 

In  the  female,  the  ovaries  are  attached  at  about  the  same  place. 


36  APPLIED  BIOLOGY 

The  oviducts  are  a  pair  of  long,  much-coiled,  white-colored  tubes, 
lying  close  to  the  dorsal  wall  of  the  body-cavity  and  at  the  sides  of 
the  kidneys.  Posteriorly  they  open  into  the  terminal  part  (cloaca) 
of  the  intestine,  while  anteriorly  they  have  funnel-shaped  openings 
into  the  body-cavity  dorsal  to  the  liver.  There  is  no  direct  connec- 
tion between  the  ovary  and  the  oviduct,  but  the  ova,  when  ma- 
ture, fall  into  the  body-cavity,  pass  through  the  above-mentioned 
openings  into  the  oviducts,  and  then  to  the  exterior  through  the 
cloaca. 

The  fat-bodies  are  tufts  of  bright-yellow  masses  attached  to  the 
dorsal  side  of  the  body-cavity  behind  the  liver.  The  fat  is  food 
stored  for  use  in  the  early  spring.  Such  fat-bodies  are  found  only 
in  frogs  and  their  near  relatives,  but  other  animals  store  fat  in 
various  organs. 

Nervous  System.  —  (D  or  L)  Remove  all  the  organs  which  have 
been  studied  and  cut  aw  y  the  floor  of  the  mouth.  Notice  (I)  the 
skull,  which  is  covered  on  its  ventral  surface  by  the  roof  of  the  niouth, 
and  (2)  the  backbone  or  vertebral  column.  The  skull  contains  the 
brain,  and  the  backbone  is  a  tube  which  incloses  the  spinal  cord. 
Looking  at  the  body-cavity  side  (i.e.,  ventral),  notice  the  large 
nerves  which  extend  from  the  vertebral  column  to  the  fore  and  hind 
limbs ;  also  some  small  nerves  extending  out  to  the  body-wall  'of 
the  back.  Examine  brains  which  have  been  hardened  by  chemicals 
and  then  removed,  and  also  observe  a  specimen  of  a  frog  dissected 
from  the  dorsal  side  to  show  brain  and  spinal  cord. 

The  bones  (skeleton)  of  the  frog  may  be  studied  later  in  compari- 
son with  those  of  some  other  backboned  animals.  The  chief  bones 
of  the  frog  can  be  identified  by  comparing  a  skeleton  with  Fig.  4. 

The  muscles  of  the  frog's  legs  should  be  examined  as  to  their 
attachments  to  the  bones.  Also,  note  how  shortening  of  muscles 
would  affect  the  movements  of  bones  to  which  they  are  attached. 
There  are  other  muscles  in  the  body- wall,  and  in  the  walls  of  stomach, 
intestine,  and  blood-tubes. 

37.  Organs  of  Frog.  —  Summarizing,  we  have  found  the 
frog  to  be  made  up  of  parts  or  organs  as  follows  :  - 

Alimentary  organs :  mouth,  mouth-cavity,  pharynx, 
esophagus,  stomach,  small  intestine,  large  intestine  (includ- 
ing cloaca),  liver,  and  pancreas. 

Breathing  organs :  nostrils,  mouth-cavity,  pharynx, 
trachea,  lungs,  and  skin. 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          37 

Circulation  organs:  heart,  arteries,  veins,  capillaries, 
lymph- vessels,  and  spleen. 

Excretory  organs:  kidneys,  ureters,  bladder,  cloaca, 
lungs,  and  skin. 

Nervous  organs :  brain,  spinal  cord,  nerves,  eye,  ear,  and 
nose. 

Supporting  organs  :  skeleton  (bones  and  cartilages) . 

Muscle  organs  :  muscles  of  body-wall,  of  limbs  (for  locomo- 
tion), and  of  internal  organs  (heart,  stomach,  etc.). 

Reproductive  organs  :  ovaries,  spermaries,  ducts  of  ovaries 
and  spermaries,  and  fat-bodies  (peculiar  to  frog  and  its 
relatives  or  allies). 

THE  TISSUES  OF  THE  FROG  :    INTRODUCTION  TO 
MICROSCOPIC   STUDY 

38.  We  have  found  that  the  frog's  body  is  composed 
of  many  parts  which  we  call  organs,  examples  of  which  are 
skin,  liver,  stomach,  heart.  We  shall  now  examine  more 
carefully  the  structure  of  these  organs. 

(L)  Examine  a  frog's  leg.  First  on  the  outside,  notice  the  skin. 
Remove  the  skin,  and  the  muscles  are  brought  into  view.  Compare 
a  piece  of  skin  with  a  piece  of  muscle.  Is  there  any  difference? 
In  separating  the  skin  from  the  muscle,  or  the  muscles  from  each 
other,  notice  fine  but  strong  threads  binding  them  or  connecting 
them  together.  Notice  bright,  glistening  bands  (called  tendons) 
which  unite  muscles  to  bones.  Next,  examine  the  nerves  which  lie 
between  the  large  muscles.  Scrape  the  muscle  from  the  bone  and 
attempt  to  cut  into  it  about  the  middle  and  also  at  the  rounded 
ends.  Is  there  any  difference  ? 

In  thus  examining  the  frog's  leg  we  find  that  it  is  com- 
posed of  several  distinct  kinds  of  building  materials.  These 
are  tissues.  The  skin  is  an  example  of  covering  and  protec- 
tive tissue  which  is  called  epithelium  or  epithelial  tissue. 
The  strong  threads  binding  skin  to  muscle,  muscles  to  each 
other,  and  (in  form  of  tendons)  muscles  to  bones,  is  known  as 


38  APPLIED  BIOLOGY 

connective  tissue.  The  muscles  contain  muscular  tissue,  and 
the  nerves  have  nervous  tissue.  The  hard  part  of  bones  is 
bony  tissue,  and  the  softer  tissue  at  the  ends  of  bones  is  called 
cartilage.  In  the  frog's  leg,  then,  we  find  epithelium,  con- 
nective tissue,  muscular  tissue,  nervous  tissue,  bone,  and 
cartilage. 

(L)  Examine  your  own  hand.  Skin  or  epithelial  tissue  covers 
and  protects  it.  Pull  up  on  the  skin,  and  it  is  found  to  be  bound 
(by  connective  tissue)  to  the  underlying  muscles,  which  you  can 
feel  and  move.  You  can  also  feel  bones.  Lastly,  you  have  the 
sense  of  touch  or  feeling  in  the  hand ;  this  indicates  the  presence  of 
nerves.  Name  the  tissues  which  you  find  in  your  hand.  Do  you 
find  any  which  were  not  seen  in  the  frog's  leg  ? 

The  tissues  which  have  just  been  examined  are  the  kinds 
of  building  materials  which  form  not  only  the  frog's  legs, 
but  also  its  whole  body.  The  same  kinds  are  also  in  our  own 
bodies.  If  we  were  to  examine  any  organ  in  the  frog  or  in 
the  human  body,  we  should  find  it  made  up  of  two  or  more 
of  the  tissues.  For  example,  the  heart  is  largely  composed 
of  muscular  tissue,  but  it  has  nerves,  connective  tissue,  and 
epithelium;  and  the  stomach  has  epithelium,  muscles, 
nerves,  and  connective  tissue.  Any  backboned  animal's 
body  is  made  up  of  many  kinds  of  materials  or  tissues  which 
have  different  appearances  and  serve  different  purposes. 
And  just  as  the  materials  (iron,  stone,  brick,  wood,  etc.) 
used  in  building  houses  may  be  put  together  in  various  com- 
binations so  as  to  form  many  different  kinds  of  buildings  for 
different  purposes,  so  the  few  kinds  of  building  materials 
or  tissues  of  an  animal's  body  are  united  to  form  organs  which 
are  quite  different  in  appearance  and  purpose.  A  frog's 
heart  does  not  resemble  a  leg  muscle  and  their  purposes  and 
work  are  different,  but  they  are  chiefly  composed  of  the  same 
kind  of  tissue  (muscular),  because  muscular  activity  is  needed 
for  movement  in  legs  and  in  the  heart.  In  like  manner,  we 
find  epithelium  wherever  there  is  a  surface,  inside  or  outside, 


AN  INTRODUCTION   TO  ANIMAL   BIOLOGY 


39 


to  be  covered;  cartilage  wherever  flexibility  combined 
with  considerable  rigidity  (e.g.,  at  ends  of  bones)  is  needed; 
connective  tissue  wherever  other  tissues  must  be  joined  to- 
gether; bones  for  supporting  framework;  and  nervous 
tissue  in  all  places  where  nervous  activity  (feeling,  sensation, 
control,  etc.)  is  needed.  Each  tissue  has  its  peculiar  purpose, 
just  as  wood,  bricks,  iron,  have  their  own  purposes.  Briefly, 
the  purposes  or  functions  of  the  tissues  are  as  follows  :  epithe- 
lium for  covering,  connective  tissue  for  uniting,  bone  for 
rigid  support,  cartilage  for  flexible  support,  muscular  tissue 
for  contraction  and  movement,  and  nervous  tissue  for  feel- 
ing and  control. 

So  far  we  have  been  studying  the  larger  structure  of  the 
frog  as  seen  by  the  unaided  eye.  We  have  been  able  to 
locate  the  various  organs  and  to  learn  something  about  their 
general  form  or  position;  but  concerning  the  structure  of 
the  organs  themselves  our  unaided  eyes  have  been  able  to 
discover  only  the  tissues.  It  is  now  necessary  to  make  use  of 
the  microscope  in  order  to  see  the  minute  structure  of  the 
tissues  which  we  find  in  organs. 

(L)  A  lesson  on  use  of  the  compound 
microscope  should  be  introduced  at  this 
point. 

39.  Cells. — (D)  Mount  a  small  piece  of 
the  outer  skin  (epithelium)  of  the  frog  in 
a  drop  of  water  on  a  glass  slide  and  cover 
with  a  cover-glass.  Examine  this  with  a 
compound  microscope  (magnification  50 
to  100).  The  epithelium  is  seen  to  be 
composed  of  small  (usually  hexagonal) 
blocks,  called  cells,  set  side  by  side  like 
bricks  in  a  wall  or  pavement  (see  Fig.  9). 
A  small  spherical  mass  (called  nucleus) 
may  be  seen  near  the  center  of  each  cell, 
and  in  most  of  the  cells  the  nuclei  may  be 
brightly  stained  by  dipping  a  piece  of  epithelium  into  a  dye  such  as 
eosin  solution  (red  ink),  then  into  water,  and  then  mounting  on  a 
glass  slide  for  microscopic  examination.  In  the  same  slide  notice 


FIG.  9.  Group  of  cells 
from  surface  epithelium 
of  frog,  s,  opening  of 
a  skin-gland  (From 
Holmes.) 


40 


APPLIED  BIOLOGY 


FIG.  10.  Upper  fig- 
ure shows  that  there 
are  many  layers  of 
cells  in  epidermis, 
those  of  Fig.  9  being 
at  the  surface.  The 


that  the  cells  are  closely  joined  together;  in 
fact,  there  is  between  them  a  cement  substance, 
which  can  be  dissolved  with  some  chemicals, 
and  the  cells  are  then  easily  separated. 

Cells.  —  We  have  chosen  the  frog's  epi- 
thelium as  a  convenient  introduction  to 
cells.  All  other  animal  and  plant  tissues 
which  biologists  have  examined  micro- 
scopically have  been  found  to  have  cells ; 
and  so  the  cells  are  regarded  as  the  units 
of  which  the  bodies  of  organisms  are  com- 
posed. Various  forms  of  cells  are  shown 
in  Figs.  10,  14,  16,  18,  and  20.  The  word 
cell  commonly  means  a  cavity,  and  it  was 


resent  two  cells  in 
surface  and  edge 
views. 


lower  diagrams  rep-  originally  applied  to  plant  cells  (e.g.}  in 
cork  and  elder-pith  which  have  cavities 
in  their  substance) ;  but  it  is  now  known 
that  most  animal  cells  and  many  plant 
cells  do  not  have  cavities.  Nevertheless,  the  word  cell  has 
become  firmly  fixed  in  biological  language,  and  so  we  must 
use  it  as  the  scientific  word  for 
the  elements  or  units  of  animal 
or  plant  tissues.  The  spherical 
structure  seen  near  the  center  of 
each  cell  of  the  frog's  skin  is  the 
most  common  form  of  nucleus; 
but  in  some  cells  of  other  ani- 
mals the  nuclei  (plural)  are  in 
various  forms — ribbon-like,  like 
a  string  of  beads,  or  even  scat- 
tered in  fragments  in  the  sub- 
stance of  the  cell.  Apparently  FlG  1L  Diagram  of  cells  cty> 

every   living   Cell  has   a   nucleus          cell-wall;  cb,  cell-body;  n,  mem- 

of    some   one   of   these   forms.       bra?e  of  nucle^s;  black  sp°ts  in 

nucleus  are  chromatm.     (From 

The  substance  of  a  nucleus  is       Verwom.) 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY 


41 


called  nuclear  material.  Careful  experiments  have  proved 
that  a  cell  cannot  continue 
to  live  without  a  nucleus. 
Organisms  grow  by  multi- 
plication of  cells,  and  the 
nucleus  plays  an  important 
part  in  that  process  of  cell-  <?"_Sf'®  g 
division.  The  boundary  of 
a  cell  is  called  cell-wall  or 
cell-membrane;  it  is  some- 
times exceedingly  delicate, 
and  sometimes  very  thick 
and  hard,  as  in  certain  plant 
cells.  The  substance  be- 
tween the  cell-wall  and  the 
nucleus  is  called  the  cell-body. 


'  ^^~ 


c.s 


*:.  V'. 

c' 

FIG.  12.  Cartilage  from  end  of  femur 
of  frog,  c,  cartilage  cells;  m,  matrix 
of  inter-cellular  substance  formed  by 
the  cells;  c.s,  empty  cell  spaces.  (From 
Parker  and  Parker.) 


Im, 


*'<• 


Cells  are  composed  of  cell-sub- 
stance, part  of  which 
is  living  substance  and 
part  stored  food  and 
other  lifeless  materials 
to  be  considered  later. 
40.  Inter-cellular 
Substance.  —  In  some 
animal  tissues  all  the 
substance  does  not  lie 
within  cell-walls ;  some 
of  it  is  between  cells  or 
inter-cellular.  In  the 
frog's  skin  and  muscles 
there  is  a  cement  sub- 
of  bone  from  leg  of  stance  between  cells 
which  holds  them  to- 
gether. This  is  called 


FIG.  13.     Cross   section 

frog.      Ic,  lacunae    or    spaces    in    which   lie 

bone  cells;    m,  marrow  cavity  in  center  of 

leg    bone;     c,    branching    tubes   from   bone 

spaces  and  containing  branches  of  the  cells;    inter.ceUular  substance, 

Im,  concentric  layers  of   bone  deposited   by  . 

the  bone  cells.     (After  Parker  and  Parker.)      and    IS    lormed    Or    SC- 


42 


APPLIED  BIOLOGY 


creted  by  the  cells.  In  an  epithelial  tissue,  such  as  the  frog's 
outer  skin,  there  is  comparatively  little  inter-cellular  sub- 
stance. In  some  tissues  there  is  a  large  proportion  of  inter- 
cellular substance,  and  an  example  will  now  be  examined. 

(D)  With  a  sharp  knife  or  razor  cut  very  thin  slices  of  cartilage 
or  "gristle"  from  the  joint  end  of  a  bone  procured  at  a  butcher's 
shop,  or  from  a  frog's  bone.  Mount  these  slices  in  a  drop  of  water 
on  a  slide.  Examine  with  a  microscope.  Notice  that  the  greater 
part  of  the  tissue  is  a  bluish-white  substance  in  which  the  cells  lie. 
The  distance  of  one  cell  from  another  is  often  more  than  the  diameter 
of  the  cells  (see  Fig.  12).  Was  this  so  in  the  epithelium  of  the  skin  ? 
It  is  evident  that  the  greater  bulk  of  cartilage  is  not  cells,  but  the 
substance  between  the  cells  (i.e.,  inter-cellular). 

A  section  of  dry  bone  is  similar  to  cartilage  in  that  there  are 
numerous  small  cavities  in  which  origi- 
nally were  located  the  cells  that  formed 
the  hard  bony  substance,  which  is  en- 
tirely inter-cellular  (Fig.  13). 

If  we  were  to 
examine  all  the 
tissues  besides 
bone  and  cartilage 
and  all  the  organs 
of  the  frog's  body, 
everywhere  we 
should  find  cells 
and  inter-cellular 
substance.  We 
may  therefore 
make  the  general 
statement  that 
the  body  of  the 
frog  is  composed 

FIG.   14.     Unstriated    muscle     °f  Cells  and  illter- 
cells  from  frog's  intestine.     Cellular  Substance. 

nit,  nucleus.  Compare  with    B  t  th    }  tt      j    formed  or  secreted 

m.c.    in    Fig.     19.      (From 

Howes.)  by  the  cells,  and  hence  the  cells  are 


FIG.  15.  Parts  of 
three  striated 
muscle  cells  from 
a  leg,  showing 
many  nuclei  (n)  in 
each  cell,  and 
blood-capillaries 
(black  lines)  be- 
tween cells. 


AN  INTRODUCTION   TO  ANIMAL  BIOLOGY 


43 


Ciliated  epi- 
thelial cells  from  the 
trachea  of  a  dog.  The 
hair-like  structures  on 
ends  of  cells  project 
into  cavity  of  the 
trachea.  They  have 
constant  lashing 
movements.  (After 
Sharpey.) 


the  primary  units  composing  the  body.     These  statements 
are  also  true  of  all  higher  animals  and  of  the  human  body. 

41.  Life-Activities  in  Cells :  Proto- 
plasm. —  Since  the  frog's  body  is  com- 
posed primarily  of  cells,  we  are  led  to 
infer  that  the  life-activities  (§  29)  are 
located  in  the  individual  cells.  For 
example,  the  shortening  of  a  muscle 
when  it  contracts  and  moves  is  the  result 
of  the  combined  shortening  of  the  thou-  FIQ  16 
sands  of  cells  of  which  the  muscle  is  com- 
posed. It  appears,  then,  that  the  living 
active  substance  is  located  in  the  cells  of 
the  body.  The  technical  word  for  living 
matter  is  protoplasm.  It  is  not  known 
how  much  of  the  cell-substance  is  living  ; 
but  it  is  certain  that  the  protoplasm  is 
the  basic  or  essential  substance  in  both  nucleus  and  cell-body. 

The  "life"  of  the 
frog,  then,  is  not 
limited  to  any  one 
organ,  such  as  the 
heart,  the  brain,  or 
the  lungs ;  rather  it 
is  located  in  every 
living  cell  of  the 
body. 

In  order  to  under- 
stand better  that  pro- 
toplasm  is  living  and 
active,  the  micro- 

FIG.  17.    Fibers  of  connective  tissue,    a,  b,  white  SCOpe  may  be  used  to 

fibers  in  bundles ;  c,  elastic  fibers.    These  fibers  examine     leaflets     of 
belong  to  inter-cellular  substance.     Numerous  j.  ,  • 

small  cells  are  present  between  the  fibers  in  ' 

connective  tissue  (see  c  in  Fig.  18) .  plants   in   which   the 


44  APPLIED  BIOLOGY 

cells  are  so  transparent  that  we  can  see  the  protoplasm  mov- 
ing (flowing)  around  inside  the  cell. 

(D)  Observe  movement  (streaming)  of  protoplasm  in  leaflets  of 
Elodea,  Nitella,  or  Chara  —  all  of  which  are  widely  distributed 
aquatic  plants.  Mount  leaflets  in  fresh  water,  and  select  transparent 
places  for  study  with  low  power  of  microscope.  In  Elodea  the  bodies 
which  contain  the  green-colored  matter  (chlorophyll)  move  with  the 
protoplasm.  Sometimes  the  movements  are  checked  temporarily  by 
the  jarring  involved  in  mounting,  and  it  is  necessary  to  let  the  slide 
stand  for  a  half-hour  or  more  before  the  protoplasm  shows  its  most 
rapid  motion. 

THE  WORK  OF  THE   ORGANS  OF  THE   FROG:    INTRO- 
DUCTION TO  ANIMAL  PHYSIOLOGY 

42.  Need  of  Food.  —  We  shall  now  consider  the  work  of 
the  organs  of  the  frog's  body,  and  first  we  may  inquire,  Why 
does  the  frog  need  food?  Some  one  may  answer,  "  To  keep 
its  body  alive/'  but  that  is  not  a  scientific  answer.  In 
studying  science  we  want  to  know  why  and  how  food  is  used 
by  the  frog  so  as  to  "  keep  going  "  the  life-processes. 

Waste.  —  In  one  of  the  first  lessons  we  noted  that  the  living 
frog  performs  a  number  of  actions,  such  as  moving  and  breath- 
ing. These  activities  result  in  a  loss  of  weight  in  the  body- 
substance.  In  other  words,  the  body  of  the  frog  behaves 
like  a  machine  in  that  all  activity  leads  to  wearing  out. 
A  new  steam-engine  begins  to  wear  out  as  soon  as  it  is  put 
into  motion,  and  at  every  working  part,  particle  by  particle, 
it  is  worn  away.  Likewise,  the  frog  is  a  machine  in  which 
wearing  out  or  wasting  is  continually  occurring  in  every  part 
of  the  body  ;  for  though  not  always  visible,  every  organ  (every 
living  cell)  of  the  body  is  working  as  long  as  the  animal  lives. 

Repair  and  Growth.  —  Now,  it  is  evident  that  this  lost 
substance  must  be  replaced  by  new  substance,  or  the  frog 
will  soon  wear  out  and  die.  In  other  words,  the  waste 
which  is  always  occurring  in  the  living  animal  must  be  made 
good  by  the  processes  of  repair.  The  materials  for  this 


AN  INTRODUCTION   TO  ANIMAL  BIOLOGY          45 

repair  are  supplied  by  the  frog's  food.  And  in  addition  to 
materials  for  repair,  food  also  supplies  those  required  for 
growth  in  size,  which  means  increase  in  number  of  cells. 

What  would  be  the  result  as  to  the  weight  of  the  body 
if  the  wasting  processes  were  more  rapid  than  those  of  repair  ? 
What  are  the  conditions  with  respect  to  the  rate  of  waste 
and  repair  in  a  young  frog  which  is  growing  rapidly? 

A  scientific  answer,  then,  to  our  question,  "  Why  does  the 
frog  need  food?  "  is  that  food  (1)  furnishes  the  new  substances 
necessary  for  assimilation  (see  §  19)  to  replace  the  body- 
substances  which  have  been  wasted  owing  to  activity ;  and  (2) 
in  addition  to  repairing,  food  may  furnish  the  materials  for 
growth.  Especially  do  young  animals  need  food  for  growth. 

Energy.  —  We  may  look  in  another  way  at  the  question, 
"Why  does  the  frog  need  food?"  The  activities  of  the 
frog  —  breathing,  eating,  moving,  etc.  —  indicate  that  the 
animal  is  continually  doing  work  or  using  energy.  Energy 
is  the  capacity  for  doing  work.  For  example,  the  energy 
of  the  coal  may  move  the  steam-engine,  the  engine  may  run  a 
dynamo  and  produce  electrical  energy,  and  the  electricity 
may  run  a  trolley-car. 

Now,  it  has  been  discovered  that  energy  cannot  be  created 
by  any  machine,  plant,  or  animal ;  but  energy  may  be  trans- 
formed. Thus  the  energy  of  the  coal  is  transformed  into 
heat  energy  and  the  mechanical  energy  of  the  engine,  the 
mechanical  energy  is  transformed  into  electrical  energy,  and 
this  into  heat  energy  (electric  heater),  light  energy  (electric 
lamp),  or  mechanical  energy  (electric  motor).  A  steam- 
engine  does  work  and  expends  energy ;  but  it  derives  its  power 
of  doing  work  (that  is,  its  energy)  from  the  coal  burned  in  the 
furnace.  Likewise  the  frog,  being  unable  to  create  energy 
within  itself,  requires  food  as  a  source  of  the  energy  which 
is  to  be  used  in  the  life-activities.  The  food  of  the  frog 
corresponds  to  the  coal  of  the  engine,  in  that  each  supplies 
the  energy  for  its  respective  machine. 


46  APPLIED  BIOLOGY 

The  value  of  the  food  of  the  frog  or  of  the  coal  depends 
upon  the  amount  of  stored  or  potential  energy  which  each 
contains.  Thus  a  ton  of  one  kind  of  coal  may  run  a  certain 
large  engine  eight  hours,  while  another  quality  from  another 
mine  may  make  the  engine  work  just  as  hard  for  ten 
hours.  Likewise,  the  amount  of  energy  in  foods  is  highly 
variable.  Chemists  have  methods  of  burning  samples  of 
foods  and  fuels  and  determining  how  much  energy  they  are 
capable  of  furnishing ;  that  is,  how  much  stored  or  potential 
energy  they  contain.  Such  tests  of  energy  value  are  espe- 
cially interesting  and  important  in  connection  with  human 
food,  and  will  be  studied  later. 

In  addition  to  supplying  energy  for  the  life-activities,  we 
should  note  that  the  food  of  the  frog  also  supplies  the  materials 
for  repair  and  growth  of  the  frog's  body,  while  the  coal  only 
furnishes  energy  and  is  powerless  to  repair  the  continual  wear 
of  the  engine.  Here  is  the  great  difference  between  a  living 
machine,  such  as  the  frog's  body,  and  the  lifeless  engine. 
Only  the  living  machine,  the  animal's  body,  has  the  power  of 
using  food  in  order  to  repair  the  parts  wasted  by  activity. 
When  the  animal's  body  grows  old,  it  loses  the  power  of 
using  food  to  make  repairs,  and  some  of  its  essential  parts 
wear  out  and  stop  the  activity  of  the  living  machine. 

Summary  :  Uses  of  Food.  —  We  can  now  give  a  complete 
answer  to  the  question,  "  Why  does  the  frog  need  food?  " 
by  saying  (1)  food  supplies  energy,  (2)  food  supplies  the  mate- 
rials for  repairing  the  waste  of  the  living  animal  body,  and 
(3)  food  supplies  the  materials  for  increase  in  size  (growth), 
especially  in  the  case  of  young,  growing  animals. 

43.  Changes  in  Food.  —  Having  learned  that  food  supplies 
materials  for  repair,  growth,  and  energy  of  the  frog's  body, 
we  now  turn  to  trace  it  as  it  is  taken  into  the  body  and 
undergoes  the  changes  which  take  place  internally  when  it 
supplies  energy,  builds  up  the  body,  and  repairs  the  wasted 
body-substances. 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          47 

In  the  third  lesson  we  found  that  an  animal  body  contains 
water,  carbon,  gaseous  substances,  and  mineral  substances. 
In  order  that  food  may  serve  for  making  new  body-substance 
for  growth  or  repair  it  must  contain  all  these;  in  fact,  all 
the  chemical  elements  found  in  the  body. 

Dissolved  Food.  —  The  food  which  is  taken  into  the 
mouth  of  the  frog  and  thence  passes  through  the  esophagus 
into  the  stomach  is  largely  solid,  such  as  worms  and  insects. 
These  must  be  reduced  to  a  liquid  condition  before  they  can 
"  soak  through  "  or  be  absorbed  through  the  stomach  lining 
into  the  blood.  Solid  food  retained  in  the  stomach  cannot 
fulfil  its  purpose  of  supplying  the  energy  and  materials  for 
repair,  for  the  reason  that  energy  is  being  expended  and  re- 
pair is  necessary,  not  only  in  the  cells  composing  the  stomach, 
but  also  in  all  living  cells  of  all  the  organs  of  the  body,  even 
in  the  remotest  parts,  such  as  the  fingers  and  toes.  It  is 
evident  that  solid  food  cannot  be  distributed  so  widely; 
but  a  solution  (like  sugar  dissolved  in  water)  can  "  soak 
through  "  the  lining  of  the  stomach  into  tubes  (blood-tubes) 
through  which  it  can  flow  to  distant  organs  and  then  "soak" 
into  the  cells.  Hence  we  see  the  necessity  of  rendering  solid 
food  soluble  in  the  water  which  is  taken  with  food. 

(D)  A  drop  of  milk,  when  examined  with  the  microscope,  is 
found  to  contain  numerous  fat  droplets.  A  drop  of  water  containing 
some  starch  shows  the  small  particles  mixed  with  the  water.  Exam- 
ining with  the  microscope  a  drop  of  water  in  which  sugar  or  salt  has 
been  dissolved,  no  particles  of  these  substances  are  evident. 

Milk  and  the  mixture  of  water  and  starch  are  examples 
of  fluids  or  liquids,  but  they  are  not  solutions.  The  particles 
of  starch  or  fat  are  not  dissolved  (i.e.,  not  soluble)  in  the 
water.  On  the  other  hand,  the  water-and-sugar  mixture 
is  a  liquid  or  fluid,  and  it  is  also  a  solution.  It  is  clear  that 
all  liquids  are  not  solutions.  This  distinction  should  be  kept 
in  mind  for  use  in  connection  with  the  changes  which  foods 
undergo  in  the  alimentary  organs,  for  in  them  foods  are 


48 


APPLIED  BIOLOGY 


chiefly  prepared  for  absorption  by  being  made  into  a 
solution  in  water.  Even  liquid  foods  like  milk  and  soup 
must  be  dissolved  so  that  no  solid  particles  can  be  seen  when 
they  are  examined  with  a  microscope. 

44.  Digestion.  —  Some  solid  foods,  such  as  sugar  and 
salt,  readily  dissolve  (are  soluble)  in  water ;  but  most  of  the 
frog's  food  consists  of  meat  and  other  things  which  have 
to  be  acted  upon  by  certain  substances  before  they  will 
dissolve  in  the  water  which  is  taken  into  the  food-tube. 
To  this  process  of  changing  foods  and  causing  them  to  dis- 
solve the  term  digestion  is  given.  Definition :  Digestion  is 
the  preparation  of  foods  for  absorption  by  the  cells.  It  is 
chiefly  a  changing  of  foods  so  that  they  dissolve  in  water. 

In  order  to  understand  how  a  chemical  change  may  make 
an  insoluble  substance  capable  of  dissolving  (soluble)  in 
water,  try  the  following  experiment :  — 

(D)  Place  a  very  small  piece  of  marble  or  limestone  (or  a  piece 
of  zinc)  in  water.  Does  it  dissolve? 
Pour  some  dilute  hydrochloric  acid  into 
the  water,  and  repeat  from  time  to  time 
until  the  limestone  (or  piece  of  zinc) 
becomes  dissolved  in  the  water. 


From  such  experiments  we  learn 
that  some  substances  which  are  not 
soluble  in  water  will  become  soluble 
after  a  chemical  change  has  been 
produced  by  an  additional  sub- 
stance. This  is  just  what  happens 
in  the  alimentary  canal  of  the  frog. 
Small  pocket-like  tubes  on  the  inside 
wall  or  lining  of  the  stomach  and  in- 
testine (called  respectively  gastric 
(stomach)  and  intestinal  glands, 
Fig.  18),  as  well  as  the  liver  and 
pancreas,  form  or  secrete  peculiar 


FIG.  18.  Lining  membrane  of 
frog's  stomach,  g,  gastric 
gland  composed  of  epi- 
thelium; c,  connective  tissue; 
m,  muscle  coat  of  stomach. 
(From  Ecker.) 


AN  INTRODUCTION   TO  ANIMAL  BIOLOGY 


49 


fluids  (called  secretions),  which  are  poured  into  the  stomach 
and  intestine.  These,  coming  into  contact  with  foods,  change 
them  so  that  they  are  dissolved  in  water  taken  with  the  food. 
We  say  that  the  secretions  of  the  gastric  and  intestinal  glands 
and  of  the  liver  and  pancreas  digest  the  various  kinds  of  foods 
which  the  frog  eats ;  by  this  we  simply  mean  that  the  foods 
are  dissolved  and  prepared  so  that  they  can  be  absorbed  by 
a  process  of  "  soaking  through  "  the  lining  of  the  stomach  and 


FIG.  19.  Part  of  a  cross  section  of  small  intestine  of  frog,  pe,  peritoneum 
surrounding  the  intestine;  m.l,  outer  muscle  layer  (longitudinal);  m.c, 
inner,  circular  muscle  layer;  bl,  blood-vessels;  e.s,  connective  tissue  layer 
(submucosa) ;  ep,  epithelial  lining;  eg,  one-cell  glands.  (After  Howes.) 

intestine  into  the  blood-tubes  (Fig.  19).  The  nature  of  these 
digestive  secretions  and  of  their  action  on  the  various  kinds 
of  foods  will  be  taken  up  more  carefully  in  close  connection 
with  similar  processes  which  occur  in  the  human  body  (Chap- 
ter XVII) .  For  our  present  purpose  it  is  sufficient  to  point  out 
that  digestion  is  preparing  foods  for  absorption  into  the  cells. 
45.  Distribution  of  Digested  Foods.  —  We  have  noted 
reasons  why  digested  foods  must  be  distributed  to  all  parts 
of  the  body  (See  §43).  We  shall  now  consider  the  means  by 
which  this  is  accomplished. 


50  APPLIED  BIOLOGY 

Absorption.  —  If  the  frog  were  a  small  animal  with  all 
its  tissues  near  the  food-tube  or  digestive-cavity  (i.e.,  the 
stomach  and  intestine) ,  the  dissolved  food  could  be  absorbed 
through  the  walls  of  the  food-tube  into  the  surrounding 
organs.  This  is  the  case  in  some  very  small  animals  of 
simple  structure,  which  will  be  studied  later.  We  can  il- 
lustrate the  absorption  of  digested  (liquid)  food  in  such  a 
simple  animal  by  the  following  experiment :  — 

(Z))  Take  four  sheets  of  blotting-paper  (or  filter-paper)  about  4 
by  10  inches  in  size,  place  them  together,  then  roll  them  into  a  cone, 
and  fasten  with  a  pin.  Dissolve  common  salt  in  water  to  make  a 
strong  solution,  and  pour  into  the  cavity  of  the  paper  cone.  Notice 
that  the  salt  solution  is  absorbed  first  by  the  inner  layer  of  paper, 
and  then  in  succession  by  the  outer  layers,  each  layer  absorbing 
from  the  next  one  nearer  the  center.  Now  unroll  the  sheets, 
mark  the  inner  one  No.  1  and  the  outer  No.  4,  and  dry  them. 
When  dry  notice  the  crystals  of  salt  on  sheet  No.  4,  proving  that  the 
salt  in  solution  has  been  absorbed  from  sheet  No  1  to  2,  from  2  to 
3,  and  from  3  to  4. 

Some  simple  animals  have  a  cylindrical  body  made  up  of 
several  layers  of  cells  which  are  in  close  contact  similar  to 
that  of  the  layers  of  paper  in  the  above  experiment.  (See 
Fig.  98,  of  Hydra.)  In  such  simple  animals  the  cavity  in  the 
center  of  the  body  is  the  digestive  cavity  in  which  food  is 
prepared  for  absorption.  The  cells  which  line  this  cavity 
are  in  contact  with  dissolved  food  and  can  absorb  directly, 
just  as  sheet  No  1  did  in  the  above  experiment ;  and  the 
cells  which  form  the  outer  layers  must  absorb  from  the  inner 
ones,  just  as  in  the  above  experiment  the  outer  layers  of 
paper  absorbed  from  those  nearer  the  center. 

46.  Need  of  Blood  for  Distributing  Food.  —  Absorption 
directly  from  the  digestive  cavity  of  the  stomach  or  intestine 
or  both  would  be  possible  only  in  a  very  simple  animal.  If 
in  the  above  experiment  we  had  used  twenty  or  thirty  sheets 
of  paper,  we  should  have  found  that  very  little  salt  solution 
would  soak  through  to  the  outermost  layer ;  and  likewise  in 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          51 

any  animal  with  many  layers  of  cells  the  outermost  cells 
would  get  insufficient  food  if  they  had  to  depend  upon  ab- 
sorption by  layers,  as  in  the  case  of  the  experiment  with 
paper.  Obviously  absorption  directly  from  the  stomach 
and  intestine  would  not  be  possible  in  an  animal  like  the 
frog,  in  which,  as  we  have  already  learned  by  dissection, 
many  organs  are  at  some  distance  from  the  digestive  cavities 
of  the  stomach  and  intestine.  Clearly,  then,  there  must 
be  some  method  of  transporting  digested  food  from  the  diges- 
tive organs  (stomach  and  intestine)  to  all  parts  of  the 
body.  This  is  one  work  of  the  blood,  which,  as  we  have 
already  found,  flows  in  a  set  of  tubes,  leading  to  all  parts 
of  the  body.  The  details  of  the  way  in  which  blood  carries 
dissolved  food  from  the  stomach  and  intestine  to  all  parts 
of  the  body  will  be  studied  in  a  later  lesson  in  connection 
with  the  same  process  in  the  human  body.  The  important 
point  in  this  lesson  is  that  the  frog  needs  blood  and  a  mechan- 
ism (pumping  heart)  for  moving  it  in  order  to  carry  absorbed 
food  from  the  stomach  and  intestine  to  all  parts  of  the  body; 
that  is,  to  every  living  cell  where  it  can  be  used  as  a  supply 
of  energy  and  as  material  for  repair  and  growth.  We  shall 
later  find  still  other  important  reasons  why  the  frog  needs 
blood. 

47.  Changes  in  Living  Cells  :  Oxidation.  —  We  turn  now 
to  the  changes  which  occur  in  the  living  cells  everywhere 
in  the  body  of  the  frog.  As  has  already  been  stated  (§42), 
the  continuous  activity  of  the  living  animal  causes  a  wearing 
out  of  the  body-substance ;  that  is  to  say,  of  the  particles  of 
cell-substance  in  the  tissues  composing  the  body.  The  wearing 
out  changes  are  largely  due  to  the  chemical  union  of  oxygen 
with  the  cell-substances,  producing  a  kind  of  slow  combustion 
or  oxidation.  We  commonly  think  of  combustion  as  the 
rapid  burning  of  substances  in  the  open  air ;  but  essentially 
the  same  kind  of  change  may  take  place  slowly.  For  ex- 
ample, a  piece  of  iron  wire  can  be  quickly  burned  (oxidized) 


52  APPLIED  BIOLOGY 

in  a  jar  of  pure  oxygen.  But  in  the  oxygen  of  the  air,  diluted 
as  it  is  with  nitrogen,  oxidation  of  iron  is  the  very  slow  process 
of  "rusting."  Another  example  :  A  piece  of  magnesium  wire 
may  be  burned  quickly  at  a  high  temperature  (§  10) ;  but 
if  we  place  a  piece  of  magnesium  wire  in  water  kept  at  the 
ordinary  temperature,  it  will  slowly  oxidize  (turn  to  white 
powder)  on  the  surface.  Now,  the  slow  oxidation  of  the 
magnesium  wire  in  water,  or  the  familiar  rusting  of  iron, 
illustrates  the  slow  chemical  changes  which  are  continually 
taking  place  in  every  living  cell  of  animals  and  plants.  We 
emphasize  the  word  "continually,"  for  oxidation  of  the  cell- 
substances  is  invariably  associated  with  the  activities  which 
we  call  "  living  "  ;  and  as  long  as  there  is  life  in  a  cell,  oxida- 
tion is  going  on  and  particle  by  particle  the  cell-substance 
and  the  food  brought  by  the  blood  are  being  burned ;  that 
is,  they  are  being  combined  with  oxygen  to  form  new  sub- 
stances. 

Heat.  —  This  slow  burning  (oxidation)  results  in  heat, 
and  this  is  how  the  human  body  is  kept  warm.  In  frogs 
and  other  lower  animals  the  heat  thus  generated  is  lost 
rapidly,  because  the  surface  of  the  body  is  not  covered  with 
hair  or  other  structures  to  prevent  loss  of  heat,  and  so  the 
frog  is  never  perceptibly  warmer  than  the  water  which  touches 
its  skin.  We  call  it  "  cold-blooded,"  because  it  is  usually 
colder  than  the  human  body. 

48.  Oxygen  Required.  —  We  have  just  learned  that 
oxidation  is  a  process  necessary  to  the  life  of  the  cells  in  the 
frog's  body.  Oxidation  requires  oxygen ;  and  therefore 
there  must  be  in  the  cells  of  the  frog  a  continual  supply  of 
that  gas.  In  some  very  small  aquatic  animals  oxygen  is 
absorbed  from  the  surrounding  water  by  all  the  cells,  and 
there  is  no  need  of  any  special  organs  for  supplying  oxygen. 
But  in  an  animal  as  large  as  a  frog  the  cells  of  the  internal 
organs  are  so  far  from  the  external  air  and  water  that  some 
method  of  distributing  oxygen  is  required,  just  as  it  is  neces- 


AN  INTRODUCTION   TO  ANIMAL  BIOLOGY          53 

sary  that  digested  food  be  transported  to  cells  which  are 
far  away  from  the  digestive  organs.  The  distribution  of 
oxygen  is  accomplished  by  the  blood,  which  absorbs  oxygen 
from  the  air  or  water  external  to  the  body  and  then  carries 
it  to  the  internal  cells.  When  the  frog  is  under  water,  oxygen 
(a  small  quantity  of  O  is  always  mixed  with  or  dissolved 
in  water  exposed  to  the  air)  is  absorbed  by  the  blood  flowing 
through  the  blood-capillaries  in  the  skin;  but  when  the 
animal  is  on  land,  oxygen  is  absorbed  directly  from  the  air, 
partly  by  the  blood  flowing  in  the  capillaries  in  the  skin  and 
also  by  the  blood  flowing  in  the  capillaries  of  the  lungs.  In 
our  own  bodies  the  blood  absorbs  most  of  the  necessary 
oxygen  in  the  lungs.  In  fishes  the  blood  flowing  through 
the  capillaries  in  the  delicate  membranes  of  the  gills  absorbs 
oxygen  from  the  water  just  as  the  frog's  skin  absorbs  some 
oxygen  when  the  animal  is  under  water. 

(D)  Examine  specimens  of  frogs'  lungs  with  injected  blood- 
vessels, frogs'  skin  with  injected  blood-vessels,  gills  of  a  fish  injected 
to  show  blood-vessels.  (These  may  be  obtained  by  injecting  with  a 
hypodermic  syringe  or  sharp-pointed  pipette  some  colored  mixture, 
such  as  starch  and  carmine  in  water,  into  the  large  arteries  of  a 
frog  killed  with  anaesthetics.) 

49.  Excretions.  It  has  been  stated  that  the  cell-substance 
(including  foods  absorbed  by  cells)  is  continually  being  oxi- 
dized in  every  living  cell  of  the  frog's  body.  This  chemical 
change  produces  a  number  of  substances  which  are  of  no 
further  use  in  the  cells  of  the  body ;  in  fact,  they  would  be 
poisonous  if  allowed  to  accumulate.  These  waste  substances 
are  called  excretions. 

We  have  already  noted  §§12-16  that  animal  substance  may 
be  analyzed  into  water,  gas,  carbon,  and  mineral  matters; 
and  also  that  the  carbon  may  be  burned.  Something  similar 
takes  place  when  cell-substance  is  oxidized  in  the  living 
body,  and  the  chief  excretions  produced  are  of  four  kinds  ; 
water,  carbon  dioxide  (a  gas  formed  by  burning  the  carbon  of 


54  APPLIED  BIOLOGY 

cell-substance),  mineral  substances,  and  peculiar  substances 
containing  nitrogen  and  called  nitrogeneous  excretions. 
Three  of  these — water,  carbon  dioxide,  and  mineral  matter — 
are  easily  shown  to  be  present  when  animal  matter  is  heated 
and  burned  in  a  pipe  or  tube  (§  13).  Also  it  could  be  proved 
by  a  careful  test  that  one  of  the  gases  given  off  when  the  meat 
is  heated  is  ammonia ;  and  this  gas  contains  nitrogen,  which 
is  a  constituent  of  the  nitrogeneous  excretions  formed  when 
cell-substance  oxidizes  in  a  living  cell.  It  is  true,  then,  that 
all  the  substances  found  when  we  analyze  animal  matter  by 
heating  and  burning  are  also  present  in  the  excretions  formed 
by  oxidation  which  takes  place  at  the  temperature  of  the 
frog's  body.  The  chief  difference  is  that  oxidation  in  the 
test-tube  is  rapid  and  at  high  temperature  while  in  the  frog's 
body  it  is  slow  and  at  low  temperature. 

50.  Removal  of  Excretions.  —  The  excretions  formed  in 
all  the  living  cells  of  the  frog's  body  are  poisonous  if  allowed 
to  accumulate.  Hence  they  must  be  eliminated  from  the 
body.  Kidneys,  skin,  and  lungs  are  the  organs  of  excretion 
or  excretory  organs.  Dissection  of  the  frog  showed  that 
many  cells  are  at  such  a  distance  from  these  organs  of  ex- 
cretion that  the  poisonous  substances  cannot  be  absorbed 
directly  by  these  organs,  thus  making  it  necessary  that 
the  blood  should  absorb  excretions  from  the  cells,  and, 
flowing  to  the  excretory  organs,  give  up  these  excretions  to 
be  eliminated  from  the  body  of  the  animal.  In  the  frog, 
the  carbon  dioxide  is  carried  by  the  blood  from  the  cells, 
where  it  is  formed,  to  the  skin  and  lungs,  where  it  is  given 
off  to  the  air  or  to  water.  The  nitrogeneous  excretions  are 
first  absorbed  from  the  cells  by  the  blood  and  then  carried 
to  the  kidneys,  where,  along  with  water,  these  excretions  are 
removed  from  the  blood,  passed  into  the  ducts  (ureters), 
and  thence  to  the  exterior.  In  some  simple  aquatic  animals, 
the  excretions  are  absorbed  directly  from  the  cells  by  the 
water  in  which  the  animal  lives ;  and  just  as  in  the  case  of 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          55 

supplying  food  and   oxygen  to  these   animals  (§§  45,  48), 
there  is  no  need  of  blood  as  a  carrier  of  excretions. 

51.  Respiration.  —  This  is  a  term  which  has  long  been  used 
in  physiology  as  almost  a  synonym  for  the  popular  word 
"  breathing."     It  includes  two  of  the  processes  which  have 
been  described ;  namely,  taking  in  oxygen  and  giving  out  or 
excreting  carbon  dioxide.     In  most  animals  the  two  processes 
go  on  at  the  same  time  and  in  the  same  organs.     Air  taken 
into  the  lungs  supplies  oxygen  to  the  blood  and  at  the  same 
time  absorbs  carbon  dioxide  from  blood  circulating  in  the 
lungs ;    and   hence   the   lungs   are   often   called   respiratory 
organs.     Keep  in  mind  for  use  in  future  lessons  that  respira- 
tion includes  (1)  supplying  oxygen,  and  (2)  excreting  carbon 
dioxide. 

52.  Summary    of    Functions  of    the    Blood.  —  We    have 
found  that  blood  and  a  mechanism  for  its  circulation  are 
necessary  in  the  frog  for  communication  between  the  living 
cells  and  certain  organs  which  communicate  with  the  ex- 
ternal world.     As  we  have  seen,  all  cells  must  have  a  supply 
of  food  and  oxygen  and  must  get  rid  of  the  substances 
(excretions)  resulting  from  the  oxidation  of  cell-substances. 
Some  small  animals  have  all  their  cells  near  the  places  where 
food  and  oxygen  must  be  absorbed  and  excretions  eliminated ; 
and  for  this  reason  such  small  animals  need  no  blood-system. 
But  in  the  frog,  and  in  all  except  the  simplest  animals,  there 
are  cells  at  some  distance  from  the  organs  which  supply  oxygen 
and  food,  and  also  far  from  those  which  eliminate  excretions  ; 
and  in  all  such  animals  the  blood  acts  as  a  transporting 
medium  which  (1)  carries  food  and  oxygen  to  the  cells  from 
the  organs  (lungs  and  digestive  organs)  which  obtain  these 
substances  directly  from  the  exterior,  and   (2)   carries  ex- 
cretions from  the  cells  to  the  organs  (lungs,  skin,  or  kidneys) 
which  pass  them  out  of  the  body.     In  animals  higher  than 
frogs  and  reptiles  the  blood  is  also  important  in  distributing 
heat. 


56  APPLIED  BIOLOGY 

53.  Nutrition.     Fundamental  Processes.  — We  have  now 
briefly  traced  food  from  its  entrance  into  the  frog's  body 
through  the  changes  of  digestion,  absorption,  and  distribution 
to  the  living  cells.     These  cells  are  active  living  machines  re- 
quiring food   (1)   for  repairing  their  waste  and  for  growth, 
and  (2)  for  oxidizing  to  give  the  energy  which  is  expended 
in  the  activities  of  the  body.     Sooner  or  later  most  of  the 
materials  entering  the  cells  as  food  become  combined  with 
oxygen,  and  the  resulting  substances  are  excretions  of  no 
further  use  to  the  living  cells. 

All  the  changes  which  food  and  oxygen  undergo,  beginning 
with  their  reception  into  the  body  and  ending  with. their 
elimination  in  the  form  of  excretions,  involve  the  processes 
of  digestion,  absorption,  circulation,  respiration,  changes 
within  cells  (metabolism),  and  excretion.  It  is  important  to 
remember  that  all  these  processes  are  fundamental,  for  one 
is  just  as  necessary  as  another.  All  the  processes  —  taking 
of  food,  digesting  of  food,  its  absorption  by  the  blood,  its 
transportation  to  the  cells,  its  absorption  by  the  cells,  its 
use  by  the  cells,  the  supplying  of  oxygen,  and  the  removal 
of  excretions  —  all  these  processes  are  linked  together,  as 
it  were,  in  a  chain,  and  each  process  must  play  its  part  in 
the  life  of  the  body. 

54.  Need  of  Organs  Working  Together :    Coordination. — 
All   the   organs   concerned   in  the  processes  named  in  the 
paragraph  above  must  work  together,  for  if  any  organ  fails 
in  the  proper  performance  of  its  work,  the  result  is  that  the 
working  of  all  the  organs  of  the  body  is  affected.     For  ex- 
ample, if  the  heart  beats  slower,  the   blood  flows  slower, 
and  consequently  the  supply  of  food  and  oxygen  to  the  cells 
and  the  removal  of  excretions  will  be  lessened.     We  know  that 
if  the  heart  stops  beating,  or  the  lungs  cease  acting,  animals 
die  at  once ;    and  the  reason  is  that  the  cells  of  the  body 
fail  to  get  their  necessary  food  and  oxygen  and  the  poisonous 
excretions  are  allowed  to  accumulate.     We  see  then  how 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY 


57 


necessary  it  is  that  all  the  organs  concerned  in  the  nutrition 
of  the  body  should  work  together  or  cooperate.  To  accom- 
plish this,  there  are  organs  which  cause  the  coordination  (work- 
ing together)  of  all  the  organs  of  the  body.  These  are  the 
nervous  organs  of  the  body — the  brain,  spinal  cord,  and  nerves. 
55.  Nervous  Organs  for  Coordination.  —  If  we  touch  a 
living  frog  anywhere  on  its  skin,  various  muscles  of  the  body 
will  contract,  causing  the  animal  to  jump.  A  similar  result 
comes  from  suddenly  thrusting 
a  stick  before  the  frog's  eyes,  or 
from  making  a  loud  noise.  The 
same  thing  happens  when  the 
frog  sees  food,  such  as  a  worm ; 
the  muscles  contract  so  that  the 
frog  jumps  and  seizes  the  worm. 
These  are  examples  of  coordi- 
nation between  the  external  or- 
gans  (skin,  ears,  and  eyes)  and  fibers, 
the  muscles  which  move  the 
body.  Likewise,  whenever  any 
change  takes  place  in  any  inter- 
nal organs,  coordinated  changes  in  other  organs,  as  in  the  beat- 
ing of  the  heart  mentioned  above,  are  caused  by  the  nervous 
organs.  The  activities  of  the  nervous  organs  are  due  to 
the  combined  working  of  the  nerve-cells  (Fig.  20)  and  their 
fibers  which  connect  them  with  various  organs  of  the  body. 
A  large  part  of  the  nervous  organs  consists  of  connective 
tissues  surrounding  the  nerve-cells  and  their  fibers. 


A  nerve-cell  and  its 
n,  nucleus;  ac,  main  fiber 
(axis  cylinder).  The  other  fibers 
are  shorter  and  with  many 
branches  near  the  cell. 


DEVELOPMENT    OF    THE    FROG:     INTRODUCTION    TO 
EMBRYOLOGY 

56.  Reproduction.  —  So  far  in  considering  the  work  of  the 
organs  of  the  frog,  we  have  given  attention  to  the  organs 
which  are  necessary  for  the  life  of  the  individual  frog,  and 


58  APPLIED  BIOLOGY 

this  includes  all  organs  except  the  reproductive  organs.  The 
latter  are  necessary  only  for  the  continuance  of  the  frog 
species;  for,  as  we  have  already  noted,  individual  animals 
of  all  kinds  live  a  relatively  short  time.  The  work  of  the 
reproductive  organs  of  the  frog  (and  of  all  other  organisms) 
is  the  production  of  new  individuals ;  and  to  the  general 
facts  in  this  line  we  shall  now  give  some  attention. 

57.  Egg-cells  and  Sperm-cells.  —  In  the  study  of  the 
structure  of  the  frog,  we  have  noted  the  position  and  form 
of  the  essential  reproductive  organs  (ovaries  and  spermaries) 
and  their  ducts  by  which  egg-cells  (from  ovaries)  and  sperm- 
cells  (from  spermaries)  are  conducted  out  of  the  body  into 
the  water  in  which  the  animals  live.  From  the  ovaries  and 
spermaries  of  frogs  the  egg-cells  and  sperm-cells  are  dis- 
charged in  early  spring.  The  eggs  develop  in  the  water, 
and  hatch  as  tadpoles.  Then,  after  a  few  weeks  or  many, 
depending  upon  the  species,  they  develop  legs,  lose  their 
tails  by  a  process  of  absorption,  and  become  small  frogs. 
This  change  from  tadpoles  into  frogs  is  called  metamor- 
phosis. 

The  ovaries  in  a  young  frog  are  masses  of  tissue  with  numer- 
ous small  egg-cells.  Each  egg-cell  is  a  spherical  mass  of 
protoplasm  with  a  nucleus  near  its  center.  As  the  eggs 
grow  larger,  each  one  accumulates  granules  of  a  material 
known  as  yolk;  and  after  a  time  the  yolk  comes  to  occupy 
one  hemisphere  of  the  egg,  while  the  protoplasm  is  concen- 
trated in  the  other.  Frogs'  eggs  examined  soon  after  they 
are  laid  in  water  are  seen  to  be  black  (with  pigment)  in  one 
hemisphere  and  whitish  (due  to  yolk)  in  the  other.  The 
black  hemisphere  contains  most  of  the  protoplasm.  Each 
egg  is  surrounded  by  an  envelope  of  transparent  jelly, 
which  was  secreted  by  the  cells  of  the  oviduct  as  the  egg 
passed  from  the  ovaries  to  the  exterior. 

The  sperm-cells  have  the  form  shown  in  Fig.  21,  A.  The 
thickened  part  is  chiefly  nucleus.  The  tails  of  living  sperm- 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          59 

cells  vibrate  rapidly  and   enable  the  cells  to  swim  in  the 
water  into  which  they  are  discharged. 

58.  Fertilization.  —  Before  an  egg-cell  can  develop  into 
a  tadpole  it  must  be  entered  by  a  sperm-cell.  Soon  after 
the  egg-cells  of  a  frog  are  discharged  into  the  water  they  are 
approached  by  the  swimming  sperm-cells ;  and  each  egg  is 
entered  by  a  sperm-cell.  The  sperm-cell  then  becomes  a 
second  nucleus,  called  sperm-nucleus.  This  nucleus  ap- 


FIG.  21.  Diagrams  to  illustrate  fertilization  of  an  egg-cell  by  a  sperm-cell. 
A,  e,  nucleus  of  matured  egg-cell;  s,  a  sperm-cell  ready  to  enter.  B, 
sperm-cell  entered  and  transformed  into  sperm-nucleus  (s).  C,  sperm- 
nucleus  and  egg-nucleus  united,  fertilization  complete.  D,  division  lead- 
ing to  two-cell  stage  (compare  with  Fig.  22,  C).  The  nuclei  are  represented 
as  having  only  two  chromosomes;  but  those  of  most  sperm-  and  egg-cells 
have  more. 

preaches  that  of  the  egg-cell,  and  the  two  nuclei  unite. 
When  this  has  occurred  the  egg-cell  is  said  to  be  fertilized, 
and  the  entering  of  a  sperm-cell  and  its  union  with  the 
nucleus  of  the  egg-cell  is  called  fertilization  (see  Fig.  21). 

59.  Division  of  the  Egg-Cell.  —  As  soon  as  the  egg-cell  is 
fertilized,  it  prepares  for  division  into  two  cells.  Delicate 
fibers  appear  and  form  a  spindle-shaped  structure  with  a 
starlike  structure  (aster)  at  either  end  (Fig.  22).  In  the 
center  of  this  spindle  are  a  number  of  small  bodies  known  as 
chromosomes.  These  are  masses  of  granules  such  as  may  be 
seen  in  any  stained  nucleus  (Fig.  11).  Within  a  few  minutes 
the  changes  shown  in  Figs.  A,  B,  C,  D  occur.  These  con- 
sist first  of  a  division  of  the  chromosomes  (Figs.  C,  D),  then 
a  division  of  the  cell-body,  and  finally  the  formation  of  a 


60 


APPLIED  BIOLOGY 


new  nucleus  containing 
the  chromosomes  in 
each  of  the  two  cells 
formed  by  the  division. 
In  warm  spring 
weather  the  first  di- 
vision is  usually  com- 
pleted within  two  hours 
after  the  egg-cells  are 
laid  in  the  water,  and 
the  egg  is  then  in  the 
two-cell  stage  (Fig.  23). 
Each  cell  appears  ex- 
actly like  the  original 
egg-cell,  but  is  one  half 
the  size.  In  another 
hour  each  cell  of  the 
two-cell  stage  will  have 
gone  through  the  same 
processes  of  division, 

FIG.  22.     Diagrams  illustrating  division  of  a     and  the  four-cell   stage 
cell.     A,  resting  cell  with  nucleus  (n)  and         -11  i       rpaPhf>H 
centrosome    (c).     B,    preparing  to  divide, 

two  asters  (a)  near  nucleus,  each  with  a  Such  divisions  of  Cells 
centrosome,  chromatin  becoming  massed 
into  chromosomes.  C,  two  asters  have 
formed  a  spindle  with  chromosomes  (ch)  in 
center.  D,  each  chromosome  divided  and 
two  halves  being  moved  toward  the  asters.  .  , 

E,  chromosomes    forming    the    two    new     eight,    Sixteen,    and 
nuclei,  and  cell-body  beginning  to  divide, 

F,  division  complete,  two-cell  stage,  each 
cell  has  the  same  structure  as  the  one  cell 
in  A.     cw,  new  cell-wall.     Compare  with 

dividins  in  Figs'  40>  41- 


OCCUr  about  every  hour  ; 

.  J 

and    SO,    in    SUCCCSSlOn, 

stages      of     two       four 


thirty-two       Cells       are 

t  i  /!?•„  oo\      T    4 

^Tmed  (S  Ig.  23).    Later 

some    cells   may  get 

slower  in  division'  and 

hence  the   stages   only 

approximate  64,  128,  256  and  more  cells,  multiplying  by  two 
when  all  the  cells  have  divided.  Usually  within  two  or 
three  days  the  egg  has  reached  a  stage  consisting  of  a  spherical 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY 


61 


mass  of  many  hundred  cells ;  and  it  is  then  ready  to  arrange 
the  cells  so  as  to  form  the  body  of  a  tadpole. 

60.  Later  Development.  —  The  changes  in  external  form 
of  developing  frogs'  eggs  should  be  observed,  either  in  eggs 
preserved  in  formalin-solution,  or  in  living  eggs  (only  in 
early  spring)  kept  in  aquaria  (e.g.,  fruit-jars)  and  examined 
daily  during  development.  In  brief,  the  changes  are  as 
follows  :  The  spherical  mass  of  cells  formed  by  the  numerous 


FIG.  23.     Stages  in  division  or  cleavage  of  frog's  egg.     The  figures  indicate 
the  number  of  cells.      (From  Thomson,  after  Ecker.) 

divisions  of  the  egg-cell  becomes  elongated,  the  various 
organs  are  formed  by  complicated  changes  which  cannot  be 
described  in  the  limited  space  of  this  book,  and  gradually 
the  embryo  becomes  a  tadpole.  Finally,  the  tadpole 
hatches,  that  is,  it  breaks  through  the  jelly  that  has  sur- 
rounded the  egg  throughout  the  development. 

The  tadpole  at  hatching  appears  larger  than  a  fertilized 
frog  egg;  but  if  dried  would  not  weigh  more,  for  no  food 
can  be  taken  until  after  hatching.  The  larger  size  of  the 
tadpole  as  compared  with  the  egg  is  due  to  water  absorbed 
during  development.  5 

The  time  necessary  for  a  fertilized  egg-cell  to  develop  into 
a  free-swimming  tadpole  varies  with  the  temperature  of  the 
water.  Also,  the  eggs  of  some  species  develop  faster  than 


62  APPLIED  BIOLOGY 

others;  toads'  eggs  will  hatch  within  three  or  four  days 
after  being  laid  and  those  of  wood-frogs  develop  almost  as 
rapidly. 

61.  Metamorphosis.  —  The  time  necessary  for  a  tadpole 
to  become  full  grown  (Fig.  24)  and  to  metamorphose  into 
a  toad  or  a  frog  varies  with  species,  and  with  food  and  temper- 
ature of  water.  Toad  and  wood-frog  tadpoles  may  metamor- 
phose within  one  or  two  months  after  hatching.  Such 
rapid  development  is  important,  for  these  animals  often  lay 
their  eggs  in  temporary  pools  which  become  dry  in  late 
spring  or  early  summer.  Bull-frogs  and  some  green  frogs 

may  live  in  the  tadpole  stage 
at  least  a  year,  for  in  early 
spring  there  are  in  permanent 
ponds  many  large  tadpoles 
FIG.  24.  Full-grown  tadpole,  m,  showing  no  signs  of  develop- 


mouth;  n,  nostril;  e,  eye;  g,  gill-slit;    jng  legs  and  shortening  tails, 
a,  anus;  Z,  leg  beginning  to  develop. 

(From  McMurrich.)  and  these  are  at  least  one 

year  old.  Probably  some  of 

these  will  become  frogs  in  the  second  summer  when  somewhat 
over  one  year  old,  while  others  may  live  still  another  year  in 
the  tadpole  stage. 

The  nature  of  the  metamorphosis  of  a  tadpole  into  a  frog 
is  popularly  misunderstood.  The  tadpoles  are  said  to  "  lose 
their  tails,"  and  this  is  often  taken  to  mean  that  the  tails 
drop  off.  This  is  not  true.  In  collecting  tadpoles  in  ponds 
one  often  finds  specimens  with  short  stubby  tails  and  well- 
developed  legs,  and  others  showing  various  intermediate 
stages  between  these  and  tadpoles  with  long  tails  and  no 
legs.  The  explanation  of  these  different  conditions  is  that 
after  the  legs  develop  the  tail  is  absorbed.  First,  the  tissues 
inside  the  end  of  the  tail  are  disintegrated  by  the  white 
blood-cells,  which  "  eat  "  the  particles  of  tissue  and  carry 
them  back  into  the  body.  In  a  short  time  after  this  process 
starts,  the  tip  of  the  tail  appears  withered.  The  white 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          63 

blood-cells  continue  this  process  of  disintegration  and  re- 
moval of  tissues  until  the  tail  has  entirely  disappeared. 
Thus  instead  of  the  tail  dropping  off,  as  is  commonly  believed, 
its  substance  is  gradually  carried  back  into  the  body  by 
internal  processes  of  absorption,  and  the  materials  are 
used  in  building  other  tissues. 

62.  Embryonic  and  Larval  Development.  —  The  develop- 
ment of  any  fertilized  egg  to  the  hatching  stage  is  called  em- 
bryonic development ;    and  the  scientific  study  of  such  stages 
of  the  frog  and  other  animals  is  animal  embryology.     The 
tadpole  is  often  called  a  larva,  and  its  changes  constitute 
larval  development. 

All  species  of  backboned  animals  have  embryonic  develop- 
ment ;  but  a  large  number  do  not  have  a  larva,  for  their  eggs 
develop  directly  into  minature  animals  resembling  the  adults. 
For  example,  it  is  well  known  that  birds  are  hatched  as 
small  birds,  and  common  mammals  at  birth  show  the  charac- 
teristics of  their  species.  All  young  animals  which  at  hatch- 
ing are  quite  unlike  their  parents  (e.g.,  as  tadpoles  are  unlike 
frogs,  and  caterpillars  are  unlike  butterflies)  are  said  to  have 
a  larval  development. 

63.  Oviparous  and  Viviparous  Development.  —  All  such 
animals  as  frogs,  birds,  turtles,  fishes,  etc.,  which  lay  eggs 
that  develop  outside  the  body  of  the  animal  that  produced 
them,  are  called  oviparous.     Such  eggs  are  usually  protected 
by  special   coverings   or   shells.      We   shall  see  later  that 
such  oviparous  development  is  very  common  in  the  animal 
kingdom. 

In  many  species  of  animals  eggs  are  retained  in  the  oviducts 
or  other  specialized  cavities  until  embryonic  development 
is  completed,  and  the  young  animals  are  "  born  alive  " ;  by 
which  we  mean  that  as  organisms  ready  for  an  independent 
existence  they  are  expelled  from  the  cavity  in  which  embry- 
onic development  occurred.  Such  internal  development  is 
viviparous.  It  occurs  in  all  mammals,  except  the  duck- 


64  APPLIED  BIOLOGY 

bills  of  Australia ;  in  some  snakes ;  in  some  salamanders ; 
in  some  fishes ;  in  some  insects  ;  and  in  other  kinds  of  lower 
animals. 

There  is  a  great  advantage  in  viviparous  over  oviparous 
development  in  the  protection  afforded  the  eggs  and  em- 
bryos. Hence  relatively  few  eggs  need  be  produced.  It  is 
well  known  that  many  fishes  and  other  oviparous  animals 
produce  an  astounding  number  of  eggs,  and  that  vast  numbers 
of  the  eggs  and  young  are  destroyed.  Sharks  illustrate 
the  advantage  of  viviparous  development  for  fishes.  Few 
eggs  are  formed,  and  these  are  retained  in  the  oviducts, 
not  only  until  the  young  animals  are  fully  formed,  but  foi; 
many  months,  until  they  have  grown  to  be  several  inches 
long.  They  are  then  born  (expelled  by  muscular  contraction 
of  the  oviducts),  and  are  well  able  to  shift  for  themselves. 
Thus  a  few  sharks'  eggs  well  protected  during  development 
will  perpetuate  the  species  as  successfully  as  would  hundreds 
of  eggs  forced  to  develop  oviparously  and  exposed  to  attacks 
of  numerous  enemies. 

A  similar  case  is  found  among  salamanders,  which  are 
near  relatives  of  the  frogs  and  toads.  Our  common 
salamanders  lay  each  spring  large  masses  of  eggs.  In  a 
species  which  lives  in  the  mountains  in  Europe,  each  female 
forms  but  two  eggs  in  a  year,  and  these  are  retained  in  the 
oviducts  until  developed  into  young  salamanders  ready  to 
care  for  themselves. 

RELATION  OF  FROG  TO   OTHER  ANIMALS:    CLASSIFI- 
CATION 

64.  Classification  of  the  Frog.  —  By  this  phrase  is  meant 
the  position  of  the  frog  in  the  scientific  list  of  the  animals 
of  the  animal  kingdom.  In  a  dictionary  or  encyclopedia 
we  find  frogs  placed  according  to  alphabetical  order,  but  in 
a  book  of  zoology  they  are  grouped  with  the  other  animals 


AN  INTRODUCTION  TO  ANIMAL  BIOLOGY          65 

which  are  very  similar  in  structure.  This  similarity  is  be- 
lieved to  indicate  close  relationship,  and  hence  the  animals 
which  are  most  like  frogs  are  said  to  be  relatives  or  allies. 

The  nearest  allies  of  frogs  (that  is,  the  animals  most  like 
them)  are  the  toads.  Next  nearest  are  the  animals  known  as 
newts  and  salamanders.  Toads,  frogs,  newts,  salamanders, 
and  similar  animals  together  constitute  a  group  of  animals 
which  have  closer  resemblances  to  each  other  than  to  any 
fishes,  reptiles,  or  other  backboned  animals.  To  this  group 
the  term  Amphibia  is  applied ;  and  we  may  speak  of  frogs 
and  salamanders  as  amphibians,  just  as  we  say  that  salmon 
and  perch  are  fishes. 

The  various  kinds  of  common  frogs  are  known  by  double 
scientific  names,  such  as  Rana  catesbiana  (bull-frog),  Rana 
sylvatica  (brown  wood-frog),  Rana  virescens  (the  leopard 
frog),  and  so  on  for  many  other  kinds.  The  common  Ameri- 
can toad  is  scientifically  known  as  Bufo  Americanus,  while 
certain  little  tree-toads  are  specimens  of  Hyla  versicolor. 

The  system  employed  in  giving  scientific  names  to  animals 
and  plants  will  be  explained  in  Chapter  VII. 

The  Life-Activities  of  a  Plant.  —  Having  now  studied  in 
considerable  detail  the  structure  and  life-activities  of  an 
animal  which  illustrates  in  a  general  way  the  life  of  all 
animals,  it  will  be  interesting  to  make  a  parallel  study  of  a 
plant  in  order  to  see  how  it  performs  the  functions  of  moving, 
breathing,  using  food,  reproducing,  etc.,  —  life-activitiee 
wh\ch  have  been  pointed  out  in  §  29  as  occurring  in  both 
animals  and  plants.  Such  a  study  is  presented  in  the  next 
chapter. 


CHAPTER  V 

STRUCTURE   AND    LIFE    OF    A   PLANT:   AN    INTRO- 
DUCTION   TO    PLANT    BIOLOGY* 

65.  Introductory  Study  of  a  Plant.  —  The  lesson  on  "  Life- 
activities  of  a  Plant  "  (§§  24-28)  has  already  called  our 
attention  to  various  processes  which  are  characteristic  of 
living  plants.  They  move,  use  food,  breathe,  reproduce, 
and  perform  other  functions  much  as  do  animals.  In  order 
to  understand  how  plants  are  able  to  carry  on  these  life-pro- 
cesses we  shall  now  enter  upon  a  careful  study  of  a  plant 
selected  because  it  illustrates  the  structure  and  work  of 
other  plants.  We  shall  see  later  that  much  of  what  is 
learned  from  the  study  of  one  plant  applies  to  all  plants, 
and  that  a  knowledge  of  plants  helps  us  to  understand  many 
things  in  the  somewhat  similar  life-processes  of  animals. 

The  word  "  plant  "  usually  calls  to  mind  familiar  trees, 
garden  vegetables,  and  ornamental  plants  ("  flowers "). 
In  fact,  most  of  the  plants  known  to  those  who  have  not 
studied  botany  belong  to  the  highest  groups,  which  are  often 
galled  the  flowering  plants.  But  the  kingdom  of  plants  is 
not  limited  to  the  flowering  plants ;  f  for  in  addition  it  includes 
a  vast  variety  of  forms  known  in  popular  science  as  flowerless 
plants.  Examples  of  these  latter  which  will  be  studied  in  this 
book  are  ferns,  mosses,  toadstools  and  mushrooms,  sea-weeds, 
molds,  yeasts,  and  bacteria.  Some  of  these  are  of  micro- 


*  To  THE  TEACHER  :  This  chapter  may  be  studied  before  the  preceding 
chapter  on  the  frog,  if  there  are  reasons  for  beginning  with  plant  study. 
See  note  in  "Teachers'  Manual,"  Chapter  V. 

t  It  is  pointed  out  later  that  seed-plants  is  a  better  name  for  the  highest 
group  of  plants. 

66 


AH  INTRODUCTION   TO  PLANT  BIOLOGY  67 

scopic  size,  and  all  are  in  general  appearance  quite  different 
from  the  familiar  flowering  plants ;  but  in  many  important 
respects  there  is  much  similarity.  It  is  therefore  possible  by 
a  careful  study  of  some  common  flowering  plant  to  learn 
many  interesting  and  useful  facts  which  are  common  to 
plants  in  general. 

The  common  bean  plant  has  been  selected  for  this  begin- 
ning study,  because  it  can  be  conveniently  grown  from  seed 
planted  in  gardens  or  in  boxes  in  the  schoolroom,  and  be- 
cause the  seeds  themselves  are  large  and  easy  to  study. 
Many  other  plants  might  have  been  selected  for  these  lessons  ; 
in  fact,  the  following  account  of  the  structure  and  life  of  the 
bean  plant  will  serve  as  a  general  guide  for  the  study  of 
any  flowering  plants,  provided  the  student  has  specimens 
of  the  plants  and  uses  this  book  chiefly  as  a  guide  to  examin- 
ing the  actual  plants. 

STRUCTURE   OF  A  BEAN  PLANT* 

66.  Organs  of  the  Plant.  —  If  we  examine  a  fully  de- 
veloped bean  plant,  we  notice  first  that  it  has  several  parts 
or  organs.  These  are  roots,  stem  with  its  branches,  buds, 
leaves,  flowers,  and  sometimes  fruits  with  seeds.  We  call 
these  organs,  because  an  organ  of  a  plant  or  of  an  animal  is  a 
structure  for  doing  a  particular  work ;  for  example,  lungs  are 
organs  for  breathing.  The  roots,  stem,  leaves,  and  flowers  are 
the  plant's  organs  for  taking  food,  for  breathing,  for  reproduc- 
ing —  in  short,  for  carrying  on  all  the  life-activities  which 
in  an  earlier  lesson  (§  29)  we  found  to  be  characteristic  of 
all  living  things.  How  can  the  organs  of  the  plant  perform 


*  The  most  prominent  part  of  the  subject-matter  of  this  division  is  a 
description  of  structure  ;  but,  as  in  the  case  of  the  frog,  some  general  reference 
to  the  work  of  the  various  organs  has  been  coupled  with  the  first  study  of 
their  structure.  A  later  division  of  this  chapter  presents  a  more  careful 
survey  of  the  life-activities  of  a  plant  in  a  way  similar  to  the  lessons  on  the 
work  of  the  organs  of  the  frog  in  Chapter  IV. 


68  APPLIED  BIOLOGY 

these  functions  ?  In  order  to  answer  such  a  question,  even 
in  part,  it  is  necessary  that  we  study  first  the  structure  of 
the  organs  of  a  plant.  For  this  reason  we  will  examine  the 
roots,  stem,  buds,  leaves,  flowers,  and  fruit  of  the  bean 
plant. 

The  Bean  Roots 

67.  General  Structur  of  Roots.  —  (L)  Carefully  dig  up  a  young 
bean  plant,  and  note  how  firmly  the  roots  anchor  it  in  the  soil, 
and  how  the  particles  of  soil  cling  to  the  small  rootlets.  Wash  by 
dipping  into  water.  Note  that  there  is  no  definite  boundary  between 
roots  and  stem.  The  small  rootlets  appear  to  be  attached  to  the 
blunt  lower  end  of  the  stem  and  also  to  a  central  root  which  seems 
to  be  the  downward  continuation  of  the  stem. 

Some  of  the  stem  is  below  ground.  That  all  the  parts  of 
the  plant  below  the  surface  of  the  soil  do  not  belong  to  the 
roots,  as  is  popularly  believed,  is  evident  if  we  compare  the 
roots  of  a  bean  plant  grown  from  a  seed  planted  four  inches 
deep  with  those  on  a  plant  from  a  seed  placed  one  inch  deep 
in  the  soil.  In  both  plants  the  roots  are  grouped  at  about 
the  same  place,  and  the  deeper  planted  one  has  more  than 
three  inches  of  stem  below  the  surface  of  the  soil,  but  at 
first  showing  no  roots.  After  growing  six  or  eight  weeks, 
other  roots  usually  start  from  the  stem  nearer  the  soil  sur- 
face. It  is  well  known  that  stems  of  many  kinds  of  plants  will 
form  roots  if  they  are  kept  in  contact  with  moist  soil. 

Usually  the  main  root  is  called  primary  root,  and  the 
branches  secondary  roots. 

On  roots  of  some  bean  plants  there  may  be  seen  small 
spherical  bodies,  some  of  them  perhaps  as  large  as  one  eighth 
of  an  inch  in  diameter  (Fig.  25).  These  are  root-tubercles. 
In  the  later  lessons  on  bacteria  (a  kind  of  microscopic  plants), 
these  tubercles  will  be  referred  to  as  caused  by  bacteria  and 
useful  as  makers  of  a  peculiar  kind  of  plant  food  supple- 
mentary to  that  furnished  by  manures  or  fertilizers  in  the 
soil. 


AN  INTRODUCTION   TO  PLANT  BIOLOGY 


69 


FIG.  25.     Root- 
tubercles. 


In  addition  to  the  structures  already  noted,  there  are 
on  the  roots  many  delicate  root-hairs  (Fig.  26).  Their  great 
importance  in  absorbing  water  will  be  explained  later. 
They  are  usually  so  firmly  attached  to  soil  par- 
ticles that  they  are  broken  when  the  plant  is 
lifted  from  the  soil.  It  is  well  known  to  gar- 
deners that  it  is  easy  to  transplant  many  plants 
if  one  carefully  lifts  a  mass  of  soil  so  as  not  to 
disturb  that  near  the  roots  and  thus  break  the 
delicate  root-hairs. 

(L)  Examine  a  bean  root  and  note  that  a  layer  of 
rind  or  bark  (cortex,  or  cortical  layer)  is  easily  scraped 
or  stripped  from  the  central  cylinder  of  wood.  Test 
the  strength  of  this  central  cylinder  of  a  small  root  by 
pulling  and  by  bending.  How  are  strong  roots  of 
advantage  to  the  plant?  Tear  a  root  lengthwise  and  note  the 
fibers  which  make  it  strong. 

68.    Microscopic    Examination    of    Roots.  —  (D)    Mount    on   an 
object-slide,  with  cover-glass,  some  small  roots  from  Tradescantia 

stems  kept  in  water,  or  from 
seedlings  of  bean,  radish,  clover, 
oats,  or  barley,  grown  on  moist 
blotting-paper  (§  81).  Using 
first  the  low  power,  note  (1)  a 
cap-like  structure  on  the  tip  of 
the  root  (Fig.  27),  (2)  a  some- 
what opaque  central  cylinder, 

(3)  a  transparent  rind  outside, 

(4)  the  outermost  layer  of  the 
rind   is    the  very   transparent 
epidermis. 

FIG.  26.     Cross  section  of  root,  showing  Cells.  —  Now,  using  the  high 

central  woody  cylinder;  rind  or  cortex;  power  of  the  microscope,  notice 

root-hairs  with  particles  of    adhering  ^^    the   rQot  ig   composed  of 

sod.     (After  Frank.)  ^.^    structureg>     the    cdls 

(Pig.  27).  Are  they  all  the  same  shape  ?  Many  of  the  cells  contain 
living  matter  (protoplasm),  but  some  appear  empty.  (D)  If  some 
iodine-eosin  (eosin  dissolved  in  a  solution  of  iodine)  be  drawn  under 
the  cover-glass  by  means  of  absorbing-paper  touched  to  the  edge  of 
the  glass  opposite  a  drop  of  the  stain,  the  protoplasm  in  the  cells  will 


70 


APPLIED  BIOLOGY 


FIG.  27.  Longitu- 
dinal section  of  tip 
of  barley  root, 
showing  root -cap 
which  covers  the 
rounded  end  of  the 
root. 


become  stained,  and  in  each  cell  a  small  rounded  body  will  stain 
brighter  than  the  surrounding  protoplasm.  This  body  is  the  nucleus, 
a  very  important  part  of  the  living  matter  of  the 
cell.  Each  cell  has  a  cell-wall,  which  is  the  hard 
substance  of  which  wood  is  composed.  Some 
cells  appear  to  have  only  a  cell-wall  and  to  be 
filled  with  water  or  air.  Such  cells  are  older  and 
have  lost  the  living  matter  which  originally  was 
contained  within  their  cell-walls. 

The  above  study  of  a  root  calls  attention 
to  the  fact  that  plants,  like  animals  (§  39), 
are  composed  of  units  called  cells,  and  that 
the  essential  living  substance  in  cells  is  the 
protoplasm.  Plant  cells  multiply  by  di- 
vision similar  to  that  shown  for  animal 
cells  in  Fig.  22.  Epidermis  stripped  from 
a  stem  of  Tradescantia,  and  from  the  leaves 
of  an  onion  bulb,  should  be  examined  and 

compared  with  the  epidermis  of  a  frog  (§  39).     See  also  Figs. 

40  and  41. 

The  Bean  Stem 

69.  General  Structure  of  Stem.  —  (L)  Note  the  general  shape 
of  the  stem  and  its  branches  in  a  bean  plant  three  weeks  old 
and  in  another  six  weeks  old.  The  places  where  leaves  are 
attached  are  joints  or  nodes  of  the  stem,  and  the  parts  of  the 
stem  between  the  nodes  are  internodes.  Compared  with  the  hard 
and  woody  stems  of  shrubs  and  trees,  the  stem  of  the  bean 
plant  is  soft  (herbaceous),  and  soon  after  the  formation  of  the 
pods  (fruit)  the  plant  begins  to  wither  and  soon  dies.  Hence  the 
duration  of  life  of  a  bean  plant  is  one  growing  season;  and  such 
a  plant  which  does  not  naturally  live  over  winter  is  an  annual. 
Name  five  other  annuals. 

With  a  sharp  knife,  cut  into  a  piece  of  a  bean  stem  from  a  full- 
grown  plant  and  then  peel  off  the  soft  rind  (also  sometimes 
called  cortex  or  cortical  layer).  Beneath  the  rind  lies  the  hard 
wood  (xylem),  which  constitutes  the  central  axis  of  the  stem.  Be- 
tween the  rind  and  the  wood  is  a  layer  of  very  soft  slippery  substance 
which  makes  it  easy  to  separate  rind  from  the  wood.  In  early  spring 


AN  INTRODUCTION   TO  PLANT  BIOLOGY  71 

when  the  stems  of  trees  are  growing  rapidly  and  this  layer  is  soft, 
boys  make  whistles  at  the  end  of  willow  sticks  by  pounding  the  bark 
until  it  is  loosened  from  the  wood  so  that  as  a  ring  it  can  be  slipped 
off,  and  then  certain  notches  are  cut  in  the  wood.  Also,  the 
same  soft  substance  between  the  bark  and  wood  of  one  species  of 
elm  trees  is  the  delicious  "slippery-elm  bark,"  which  in  a  dry  and 
powdered  form  is  sold  by  druggists  for  alleviating  irritations  of  the 
throat. 

In  the  center  of  the  soft  substance  between  bark  and  wood  of 
such  stems  is  a  layer  of  cells  which  are  active  in  growth  and  division. 
This  is  the  growing  layer  or  cambium.  On  its  inner  side  new  wood 
cells  are  formed  and  added  to  the  older  wood  of  the  stem,  and  on  its 
outer  side  the  new  cells  formed  are  added  to  the  inner  surface  of 
the  bark.  Hence  growth  and  division  of  cambium  cells  adds  new 
cells  to  both  wood  and  bark,  resulting  in  increased  diameter  of  the 
stem. 

On  the  outside  of  the  rind  of  the  bean  stem  is  a  thin  layer  of 
cells,  the  epidermis.  Some  cells  of  the  rind  are  green  in  color,  which 
is  due  to  a  substance  common  in  leaves  and  known  as  leaf -green  or 
chlorophyll. 

With  a  sharp  knife  or  section-razor,  make  a  cross  cut  (transverse 
section)  of  the  stem.  Examine  with  hand-lens,  and  note  relative 
thickness  of  rind  and  wood.  In  the  center  of  the  wood  is  a  softer 
substance,  known  as  pith;  and  in  the  oldest  parts  of  the  bean  stem 
there  is  a  cavity  in  the  pith. 

Split  open  lengthwise  (longitudinal  section)  a  stem  from  an  old 
bean  plant,  and  note  the  extent  of  the  pith  and  of  the  central  or 
pith-cavity.  Tear  the  wood  apart  lengthwise,  and  note  that  it  is 
"stringy."  Pay  special  attention  to  the  arrangement  of  the  wood, 
bark,  and  pith  in  a  longitudinal  section  at  a  branch.  Make  dia- 
grams showing  position  of  rind,  cambium,  wood,  and  pith. 

70.  Microscopic  Study  of  Bean  Stem.  —  (D)  With  a  razor  or  very 
sharp  knife  cut  a  very  thin  transverse  slice  or  section  of  the  stem, 
mount  in  water  on  a  glass  object-slide.  Examine  with  low  power 
first.  Examine  the  rind  and  its  outermost  layer  (epidermis),  the 
wood,  and  the  pith.  These  are  examples  of  the  tissues  or  building 
materials  of  the  plant.  Use  a  higher  power  of  the  microscope,  and 
note  that  each  of  these  tissues  resembles  a  piece  of  honeycomb  or 
hollow  blocks  set  together  like  bricks  in  a  wall.  These  are  the  plant 
cells.  Some  of  the  substance  in  the  cells  is  the  living  matter  or 
protoplasm,  but  some  cells  in  the  central  part  of  the  stem  appear  to 
be  empty.  The  fact  is  that  such  empty  cells  are  dead,  and  only  their 


72  APPLIED  BIOLOGY 

hard  cell-walls  remain.  Examine  a  stained  section  (permanent 
preparation,  or  stained  as  suggested  in  §  68.  In  some  of  the  cells 
darkly  stained  bodies  (nuclei)  may  be  seen.  These  also  are  com- 
posed of  living  matter,  and  as  far  as  is  known  every  living  plant  cell 
has  a  nucleus. 

Carefully  compare  the  section  as  seen  with  the  microscope  and 
Fig.  48.  Locate  the  thin-walled  sieve-tubes  in  the  inner  layer  of  the 
rind,  and  the  surrounding  bast-fibers.  These  tubes  and  fibers  make 
the  rind  "stringy."  The  function  of  the  sieve-tubes  is  to  conduct 
fluids  down  the  stem  (§  103). 

Note  in  the  wood  of  the  stem  certain  groups  of  large  empty  cells. 
These  are  really  groups  or  bundles  of  tubes  (wood-tubes)  and  strong 
fibers  (wood  fibers)  extending  up  and  down  the  stem.  These  are 
the  bundles  of  tubes  and  fibers  which  make  the  wood  "stringy," 
as  already  noted. 

The  tubes  and  fibers  of  the  rind  are  separated  from  those  of  the 
wood  by  the  cambium  (Fig.  48).  All  the  tubes  and  fibers  taken 
together  constitute  the  fibro-vascular  bundles,  meaning  bundles  of 
fibers  and  tubes.  They  will  be  studied  more  carefully  in  other  kinds 
of  stems  later. 

The  work  or  function  of  the  fibers  of  the  wood-bundles  may  be 
discovered  by  bending  the  stem  to  test  its  rigidity  and  by  pulling  to 
test  its  strength.  The  function  of  the  wood-tubes  is  shown  by  the 
following  experiment :  — 

(D  or  L)  Cut  off  a  young  bean  stem  and  stand  it  in  a  bottle  with 
red  ink  (eosin  solution  with  water).  After  a  half-hour,  carefully 
strip  off  the  bark  in  various  places  in  order  to  see  the  red  color  in  the 
woody  part  of  the  stem.  Also  cut  across  the  stem  in  various  places. 
It  is  obvious  that  liquids  go  up  the  stem  in  the  wood-tubes. 

The  Bean  Buds 

71.  Position  and  Kinds  of  Buds.  —  At  various  places  on 
a  bean  plant  are  buds,  which  will  unfold  later.  Some  of  these 
buds  will  form  flowers,  and  hence  are  called  flower-buds; 
others  will  unfold  as  leaves  and  are  therefore  leaf-buds.  At 
the  ends  of  the  main  stem  and  its  branches  are  terminal  buds 
which,  by  growth,  lengthen  the  stem  and  branches.  If 
the  terminal  bud  of  the  main  or  a  branch  stem  be  destroyed, 
lengthening  of  the  stem  will  cease.  For  this  reason  gardeners 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  73 

often  break  off  the  terminal  buds  of  climbing  or  pole  beans 
(e.g.,  limas)  when  they  have  reached  a  height  of  about  six 
feet.  This  has  the  effect  of  sending  into  the  developing 
fruit  (pods)  much  of  the  nutriment  which  would  have  gone 
to  make  the  useless  extension  of  the  stem. 

(L)  Examine  buds  on  a  young  bean  plant,  (1)  with  reference  to 
their  position,  and  (2)  separate  carefully  the  parts  of  both  a  leaf- 
and  a  flower-bud. 

The  Bean  Leaves 

72.   Leaf  and  Leaflets  of  Bean  Plant.  —  (L)    Brief  examination 
of  these  will  show  that  what  we  at  first  take  to  be  the  individual 
leaves   are   arranged    in  groups  of  three   (Fig.  28).      A  study  of 
the  development  of  the  bean  has 
led  botanists  to  the  conclusion 
that  the  three  leaves  are  really 
three  parts  or  divisions  of  one 
leaf.     Such  a  leaf  is  called  com- 
pound,  and  the  three  divisions 
are  called  leaflets.     The  support- 
ing leaf-stalk  (petiole)  is  attached 
at  a  node  of  the  stem  and  can 
be  easily  distinguished  from  a 
branch  of  the  stem,  because  the 

leaf -stalk  has  an  enlarged  and  night.  (From  Detmer.) 
somewhat  flexible  attachment  to 
the  stem;  and  also  because  a  longitudinal  cut  through  the  node 
shows  that  the  leaf  is  attached  chiefly  to  the  rind,  while  the  branch 
is  attached  firmly  to  the  wood  of  the  stem.  Each  of  the  leaflets  is 
attached  by  a  thickened  joint  similar  to  that  of  the  petiole  at  the 
stem.  These  flexible  joints  of  the  leaf  and  leaflets  allow  the  droop- 
ing of  the  leaves  at  night  and  on  hot  days. 

Examine  any  one  of  the  leaflets,  and  note  that  the  expanded 
surface  (blade)  is  a  thin  sheet  of  tissues  supported  by  ribs  and  veins. 
The  central  rib  is  the  mid-rib.  Hold  the  leaflet  up  so  that  light 
may  shine  through  it,  and  with  a  hand-lens  examine  the  delicate 
veins  (veinlets)  between  the  ribs.  Make  an  outline  sketch  showing 
the  structure  of  a  complete  leaf  and  the  detailed  structure  of  one  of 
the  three  leaflets  ;  and  label  all  the  parts  named  above. 


74  APPLIED  BIOLOGY 

73.  Microscopic  Structure  of  Bean  Leaf.  —  (D)  With  fine-pointed 
forceps  strip  off  a  very  small  piece  of  the  thin  transparent  epidermis 
from  both  the  upper  and  lower  surfaces  of  a  leaf,  and  mount  for 
microscopic  study.  This  will  present  surface  views  of  the  leaf. 
With  low  power,  note  the  irregular  cells  of  which  the  epidermis  is 
composed  (compare  with  Fig.  29,  which  is  from  other  leaves). 

Scattered  among  these  irregular  cells  are  pairs  of  crescent-shaped 
cells  set  together  so  as  to  leave  a  small  pore  or  opening  through  the 
epidermis.  Each  pore  is  a  leaf-pore  or  stoma  (a  Greek  word  mean- 
ing an  opening,  plural  stomata).  It  opens  into  a  small  cavity  (air- 
space) beneath  the  epider- 
mis. Are  the  leaf-pores 
present  and  equally  abun- 
dant on  both  upper  and 
lower  sides  of  the  bean- 
leaf? 

These  two  crescent- 
shaped   cells    around    the 
pore  are  called  guard-cells. 
It  has  been  noticed  that 
FIG.  29.     Epidermis  from  certain  plant  leaves.        u       watpr  ;<,  ohnnHanf  in 
A,   upper  surface.     B,  lower  surface  with    when  water  is  abundant  m 
stomata.     (From  Strasburger.)  the  plant  these  cells  swell 

and  become  more  crescen- 

tic  in  form,  leaving  a  larger  opening  than  when  the  leaf  is  drier.  It 
is  believed  by  most  botanists  that  the  guard-cells  are  able  to  prevent 
excessive  evaporation  of  water  from  the  air-spaces,  and  thus  conserve 
water  when  necessary. 

Make  a  drawing  of  a  group  of  cells  from  the  epidermis,  including 
at  least  one  pair  of  guard-cells. 

Cut  off  a  leaf  transversely,  and  with  a  strong  hand-lens  examine 
the  cut  end.  Note  the  transparent  epidermis  which  covers  both 
sides  and  the  edges ;  that  is,  entirely  surrounds  the  leaf.  Between 
the  upper  and  lower  epidermis  the  center  of  the  leaf  appears  to  be 
filled  with  a  green-colored,  somewhat  granular  material  (middle- 
tissue  or  mesophyll).  Also  notice  the  cut  ends  of  the  colorless  veins. 
If  the  hand-lens  is  a  strong  magnifier,  it  is  possible  to  see 
that  the  green-colored  middle-tissue  is  more  compact  toward  the 
upper  surface  and  appears  to  have  small  cavities  in  the  part  next 
the  lower  epidermis.  This  will  be  very  clearly  seen  when  the  com- 
pound miscroscope  is  used  for  examining  the  cut  end  of  a  leaf. 

(Z))  Examine  with  a  miscroscope  a  very  thin  transverse  section 
from  the  end  of  a  leaf  cut  like  the  one  described  above.  [To  cut 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


75 


such  a  section  hold  the  leaf  between  two  pieces  of  elder-pith,  or  slices 
of  potato,  or  roll  the  leaf  to  form  a  rather  tight  cylinder.  Keep  the 
edge  wet  with  water  and  with  a  very  sharp  razor  shave  off  a  number 
of  very  thin  slices.  Float  these  off  the  razor  into  water  and  with 


FIG.  30.  Cross  section  of  privet  leaf,  ep,  epidermis;  pi,  palisade  cells  in 
upper  part  of  middle  tissue;  A;,  deposited  crystals;  a,  air-spaces;  »,  vascu- 
lar mid-rib;  st,  a  stoma.  (From  Strasburger.) 

fine  forceps  or  a  small  brush  transfer  these  sections  to  a  drop  of 
water  on  a  glass-slide  and  place  the  cover-glass  in  position.)  Com- 
pare a  transverse  section  with  Fig.  30  and  locate  all  the  structures 
in  the  cut-end  of  the  leaf  that  was  seen  with  the  hand-lens.  Note 
that  the  epidermis  is  transparent  and  without  green  color,  except 
in  the  cells  (the  guard-cells)  at  the  leaf- 
pores.  Compare  with  the  epidermis  which  you 
previously  stripped  off  and  examined  in  sur- 
face view. 

Most  of  the  cells  in  the  middle-tissue  have 
green  bodies  (the  chlorophyll-bodies,  or  chloro- 
plasts).  The  compact  upper  part  of  this 
middle-tissue  is  seen  to  be  composed  of  elon- 
gated cells  (palisade  cells)  set  closely  together. 
In  the  lower  part  of  the  leaf  the  cells  are  irreg- 
ular in  shape  and  there  are  numerous  air-spaces. 
Some  of  these  spaces  communicate  with  the 
outside  through  the  leaf-pores,  and  thus  air 
may  enter  the  leaf  and  become  widely  distrib- 
uted throughout  the  air-spaces. 

Bundles  of  small  transparent  cells  in  the  middle-tissue  are  the 
cut  ends  of  veins,  and  the  empty  cells  in  them  are  tubes  for  carrying 


FIG.  30 a.  Magnified 
view  of  cells  around 
a  stoma  (s).  a,  air- 
space; c,  cells  with 
chlorophyll;  lower 
cells  (e)  epidermis. 
(From  MacDougal.) 


76 


APPLIED  BIOLOGY 


water.  The  significance  of  these  details  of  structure  will  be  made 
clear  in  the  next  part  of  this  chapter,  in  which  the  work  of  the  leaf 
is  described. 


THE  REPRODUCTION  OF  THE  BEAN  PLANT 

The  Bean  Flower 

74.  Simple  Flowers.  —  The  bean  flower  is  too  difficult  for  the 
first  examination  of  the  parts  of  a  flower.  However,  the  parts  of 
simple  flowers  are  usually  so  well  learned  in  the  nature-study  of 
the  elementary  schools  that  it  is  really  an  advantage  to  study  in 
the  high  school  different  types  of  flowers.  In  case  any  pupils  in 
the  class  have  not  previously  learned  the  parts  of  any  simple 
flower,  they  should  first  study  one,  identifying  :  (1)  The  calyx, 
composed  of  sepals;  (2)  corolla,  composed  of  petals;  (3)  stamens, 
composed  of  the  stalk  or  filament  and  the  anther,  bearing  minute 
grains  of  pollen ;  (4)  in  the  center  of  the  flowers  the  pistil,  composed 
of  the  ovary,  and  the  style,  which  extends  upward  and  has  the 
stigma  at  its  end.  Flowers  which  have  separate  petals  (scillas, 
tulips,  sedum,  lilies,  etc.)  are  good  for  such  pre- 
liminary study  of  the  parts  of  simple  flowers. 

75.  Study  of  Bean  Flower.  —  (L)  Bean  plants 
six  to  ten  weeks  old  will  furnish  all  the  stages 
needed  for  this  study.  Note  how  the  flower- 
stalks  (pedicels)  are  attached  at  the  nodes  of 
the  stem  and  often  in  the  axils  of  the  leaf ;  that 
is,  in  the  angle  between  leaf  and  stem.  Iden- 
tify :  (1)  two  green  leaf -like  structures  (bracts) 
at  the  base  of  the  flower.  On  very  young 
flower  buds  on  the  same  plant  these  bracts  may 
be  seen  inclosing  the  flower.  (2)  Between  the 
bracts  and  the  corolla  is  the  calyx,  composed  of 
five  sepals  united  into  a  cup.  This  can  be  seen 
best  in  an  old  flower  from  which  the  corolla  is 
ready  to  fall.  (3)  The  petals  (white,  pink,  or 
red)  of  the  corolla,  five  in  number  and  unequal 
in  size,  are  arranged  as  in  the  diagram  in  Fig. 
31.  The  three  largest  petals  are  so  prominent 
that  at  first  sight  the  flower  appears  to  have 
only  three ;  but  two  smaller  petals  are  united  and  coiled  so  as  to  lie 
between  the  three  largest  petals.  The  largest  petal  of  a  flower  like 


FIG.  31.  Pea  flower. 
«,  "  standard  "  ;  w, 
"wings";  k,  "keel"; 
—  see  text.  (From 
Gray.) 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  77 

that  of  the  bean  plant  is  called  the  "standard" ;  the  two  petals  at 
the  sides  of  the  flower  are  called  the  "wings" ;  and  the  united  and 
twisted  petals  form  the  "keel,"  which  lies  between  the  "wings."  In 
a  bud  just  about  to  open  note  how  the  largest  petal  ("standard") 
incloses  the  others. 

Inside  the  coil  of  petals  ("keel")  are  ten  stamens  and  the  pistil. 
Look  at  a  flower  in  its  natural  position  on  a  plant  and  note  that  the 
two  side  petals  (the  "wings")  are  the  only  ones  on  which  a  bee 
could  easily  alight.  If  you  have  opportunity,  watch  bees  visiting 
bean  flowers  in  the  garden.  Hold  a  flower  by  its  stalk  in  the  natural 
position,  and  with  a  pencil  press  downward  on  the  two  side  petals, 
and  carefully  watch  the  end  of  the  coiled  petals  ("keel")  as  you  press. 
Note  how  the  stigma  and  part  of  the  style  appear  when  the  side 
petals  are  pressed,  and  disappear  within  the  coiled  petals  when 
released.  Imagine  how  a  bee  could  cause  the  same  exposure  of 
the  pistil  when  alighting  on  these  petals.  Hold  these  side  petals 
down  so  as  to  keep  the  style  protruded  and  with  a  hand-lens  examine 
the  stigma,  and  also  notice  rows  of  hairs  along  the  style. 

Now,  carefully  uncoil  the  twisted  petals  and  note  how  the  style 
and  the  ten  stamens  are  inclosed  by  the  coiled  petals.  The  fila- 
ments of  nine  of  the  stamens  are  joined  together  at  their  base.  The 
stamens  are  firmly  fastened  in  place,  but  the  style  is  not  attached  to 
the  spiral  tube  formed  by  the  twisted  petals.  The  stigma  does  not 
touch  the  anthers,  but  the  hairs  just  below  the  stigma  brush  over 
the  anthers,  and  some  pollen-grains  cling  to  them. 

The  meaning  of  this  remarkable  apparatus  is  this :  When 
a  bee  alights  on  a  bean  flower  the  stigma  and  the  upper  part  of 
the  style  is  pressed  out,  as  we  have  seen,  and  pollen  is  brushed 
on  the  bee's  body  by  the  hairs  on  the  style.  Then  the  bee 
goes  to  another  flower,  and  when  its  style  touches  the  bee's 
body  the  stigma  will  touch  some  pollen  from  the  first  flower ; 
and  at  the  same  time  the  hairs  of  the  style  will  brush  on  to 
the  bee  some  pollen  from  the  second  flower.  And  so,  as  the 
bee  goes  from  flower  to  flower,  it  will  brush  pollen-dust  on 
stigmas  and  get  pollen-dust  brushed  out  from  the  anthers 
by  the  hairs  on  the  styles  below  the  stigmas.  There  is  very 
little  chance  that  the  stigma  of  a  flower  will  get  pollen- 
dust  from  anthers  in  the  same  flower.  This  may  sometimes 


78  APPLIED  BIOLOGY 

happen  when  a  bee  leaves  a  flower  and  at  once  goes  back  to 
it,  carrying  pollen-dust  received  on  its  first  visit. 

It  seems  probable  from  the  arrangement  of  the  bean 
flower  that  insect  visits  are  necessary  to  distribute  the  pollen- 
dust  of  this  kind  of  flower.  Darwin,  the  famous  English 
biologist,  made  many  experiments  by  keeping  beans  covered 
with  netting  so  that  insects  could  not  reach  the  flowers,  and 
the  result  was  that  seeds  rarely  formed.  Pea  flowers  are 
very  similar  to  those  of  the  bean,  but  the  stigma  is  so  near  the 
anthers  that  it  often  gets  pollen-dust  before  the  flower  is 
visited  by  insects.  Botanists  call  such  a  flower  self -pollinated. 
The  bean,  then,  is  not  often  self-pollinated,  but  cross-pollina- 
tion (meaning  pollen  from  other  flowers)  usually  occurs. 

We  now  see  the  significance  of  the  peculiar  irregular 
arrangement  of  the  petals  of  the  bean  flower :  the  three 
big  petals  are  arranged  so  as  to  make  bees  or  similar  insects 
alight  in  a  certain  way;  and  the  other  petals  form  a  pro- 
tective covering  for  the  stamens  and  pistil,  and  at  the  same 
time  are  a  curious  mechanism  for  preventing  pollen-dust 
from  reaching  the  stigma  of  the  same  flower,  thus  insuring 
cross-pollination. 

Many  plants  closely  related  to  the  bean  —  clover,  locust, 
pea,  wistaria,  peanut,  are  common  examples  —  have  similar 
flowers.  If  opportunity  offers,  observe  insects  visiting  such 
flowers.  There  are  many  other  types  of  flowers  arranged  or 
adapted  for  the  transfer  of  pollen-dust  by  insects,  and  some 
of  these  will  be  described  in  the  lesson  on  flowers  (§  192). 

Ovules.  —  (D)  Carefully  remove  the  corolla  and  thus  expose 
the  pistil  of  a  bean  flower.  Hold  up  to  the  light  and  notice  a  row 
of  opaque  spots  in  the  ovary.  Then  with  a  sharp  knife  or  a  razor 
split  the  ovary  lengthwise.  A  low-power  miscroscope  or  a  hand- 
lens  will  make  clear  that  the  opaque  spots  are  "seeds."  In  this 
early  stage,  however,  the  term  ovule  should  be  applied  to  each  one 
of  these  structures  which  later  grows  into  a  seed.  Inside  each 
ovule  an  embryo  develops,  and  later  when  the  seed  sprouts  the 
embryo  grows  into  a  new  plant. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


79 


Pollen-Grains.  (D  or  L)  Examine  some  pollen-grains  mounted 
in  a  drop  of  water,  using  first  low  power  and  then  high  power  of 
microscope. 

Fertilization.  —  When 
pollen-grains  have  be.en 
lodged  on  the  stigma  of  a 
bean  flower  they  soon  swell, 
and  each  one  sends  out  a 
delicate  tube,  which  grows 
down  through  the  tissues  of 
the  style  and  into  an  ovule 
in  the  ovary  (Fig.  32).  There 
the  pollen-tube  meets  a  cell 
known  as  the  egg-cell.  Then 
a  mass  of  protoplasm  (really 
a  cell)  passes  from  the  end  of 
the  pollen-tube  into  the  egg- 
cell.  The  union  of  the  two 
cells  is  fertilization.  Soon 
the  fertilized  egg-cell  divides 
into  many  cells,  and  these 
form  an  embryo  within  the 
ovule,  which  grows  into  a  seed.  This  process  of  fertilization 
in  plants  will  be  described  more  in  detail  in  the  section  on  the 
reproduction  of  flowering  plants  in  the  next  chapter. 

Pea  Flower.  —  (L)  If  material  is  available,  make  a  brief  examina- 
tion of  the  flower  of  a  pea,  preferably  of  a  sweet  pea,  in  order  to  see 
better  some  parts  which  are  larger  than  in  the  bean  flowers. 


FIG.  32.  Diagram  of  a  flower.  C,  calyx; 
co,  corolla;  a,]  anther  on  filament  (/); 
p,  pollen-grains;  st,  stigma;  pt,  pollen- 
tube;  s,  style;  0,  ovary;  em,  egg-cell 
in  ovule;  c,  fertilizing  cell.  (From 
Bessey.) 


The  Bean  Pod  or  Fruit 

76.  Fruits.  —  After  the  stigma  of  the  bean  flower  has 
been  pollinated  by  insects,  the  ovary  of  the  pistil  soon  begins 
to  develop  into  the  pod  containing  the  seeds.  Botanists 
call  the  pod  with  its  seeds  a  fruit,  and  apply  this  name  to  the 


80  APPLIED  BIOLOGY 

structures  containing  seeds  which  develop  from  flowers. 
Hence  many  things  are  known  in  biology  as  fruits  which  we 
do  not  popularly  call  fruits;  for  instance,  a  tomato,  a  bean 
pod,  a  squash,  or  a  cucumber  is  in  botany  just  as  much  a 
fruit  as  an  apple,  a  peach,  or  an  orange. 

77.  Study  of  a  Bean  Pod.  —  (L)  On  a  full-grown  bean  plant 
about  six  weeks  old  one  may  usually  find  pods  of  various  sizes, 
ranging  from  one  but  slightly  larger  than  the  pistil  of  the  flower  up 
to  the  size  of  the  fully  developed  pod.  The  stalk  of  the  pod  is  the 
same  as  the  flower-  stalk  or  pedicel  of  the  flower,  and  its  expanded 
end  at  the  point  where  it  is  attached  to  the  pod  (fruit)  is  the  recep- 
tacle, which  also  may  be  seen  beneath  the  flower.  In  the  younger 
pods,  identify  the  calyx  and  the  bracts  of  the  flower,  which  can  still 
be  seen  at  the  stem  end ;  and  at  the  other  end  find  the  style. 
Between  the  calyx  and  the  style  is  the  ovary,  which  begins  to 
elongate  shortly  after  pollination.  If  possible,  examine  the  ovary  in 
a  faded  flower,  just  about  the  time  that  the  corolla  is  ready  to 
fall  off.  Label  sketches  of  young  pods  so  as  to  show  what  parts 
of  the  flower  develop  into  the  fruit. 

Study  a  full-grown  bean  pod  (use  green  pods,  known  as  "string- 
beans").  Sketch  and  label,  naming  the  parts  by  comparing  with 
younger  pods.  The  pointed  end  is  the  base  of  the  style,  most  of 
which,  with  the  stigma,  was  pulled  off  by  the  falling  corolla.  Note 
that  the  pod  is  composed  of  two  similar  valves  fastened  together 
along  the  edges,  which  are  called  sutures  (meaning  seams).  The 
bean  pod  is  bilaterally  symmetrical.  The  position  of  the  style  and  the 
concave  curvature  of  the  pod  mark  the  edge  or  suture  where  the 
seeds  are  attached.  This  is  called  a  ventral  suture,  because  it  is  down 
or  toward  the  ground  in  the  natural  position  of  the  flower.  The 
opposite  edge  is  dorsal.  These  terms  are  applied  in  the  same  way 
to  animals,  the  upper  side  or  back  always  being  the  dorsal,  and  the 
ventral  the  opposite  side  toward  the  earth. 

Carefully  split  open  a  green  pod  along  the  dorsal  suture  and  note 
that  some  beans  (or  seeds)  are  attached  to  each  half  of  the  pod. 
Is  there  any  regularity  as  to  the  number  attached  to  either  half  ? 
Note  that  each  bean  is  held  in  place  by  a  short  seed-stalk  (funiculus). 
Undeveloped  ovules  may  be  seen  near  the  ends  of  the  pod.  The  part 
of  the  inner  lining  of  the  pod  to  which  the  seeds  are  attached  is 
known  as  the  placenta.  Make  a  labeled  sketch  of  the  opened  pod, 
showing  the  seeds  in  position. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  81 

(Optional.)  Cut  transverse  sections  of  the  pod  in  several  places, 
and  identify  the  layers  in  the  wall  of  the  pod,  the  seed-stalk,  placenta, 
dorsal  and  ventral  sutures.  Sketch  and  label. 

Break  a  pod  transversely  in  several  places,  and  notice  strong 
fibers.  On  which  edge  of  the  pod  ?  Do  you  see  any  relation  between 
the  position  of  the  strongest  fibers  and  the  natural  curvature  of  the 
pod  ?  Why  are  the  green  pods  called  "string-beans "  by  gardeners ? 

(D)  Cut  a  fresh  bean  branch  having  young  pods,  and  place 
cut  end  in  red  ink.  After  a  time  note  where  the  pods  are  colored 
and  draw  your  conclusions  as  to  the  functions  of  the  fibers  in  the 
pods.  Is  there  any  reason  why  these  fibers  should  be  more  abundant 
on  one  edge  of  the  pod? 

Look  at  specimens  of  pods  which  have  matured,  dried,  and  split 
naturally.  Does  the  splitting  (dehiscence)  usually  occur  on  the 
dorsal  or  ventral  edge,  or  both? 

Take  out  a  bean  and  note  its  markings  where  it  was  attached  to 
the  seed-stalk.  The  scar  left  when  the  seed-stalk  is  pulled  off  is 
called  the  hilum.  At  one  side  of  the  hilum  (toward  the  stalk  of  the 
pod)  is  a  small  translucent  elevation  with  a  slit-like  marking.  On  the 
opposite  side  of  the  hilum  is  a  very  minute  pit  known  as  the  mi- 
cropyle.  When  pollen-grains  touch  the  stigma,  as  previously  de- 
scribed in  §75,  the  very  small  tube  that  grows  from  each  pollen- 
grain  extends  down  the  style  and  along  the  placenta  of  the  ovary 
to  the  micropyle  of  an  ovule.  Later,  when  fertilization  is  com- 
pleted as  described  in  §  75,  each  ovule  develops  into  a  seed,  and  the 
entire  ovary  into  the  fruit  or  bean  pod.  Look  at  an  opened  bean  pod 
with  the  seeds  in  position,  and  note  whether  the  micropyle  is  above 
the  seed-stalk  or  below  (toward  the  style).  Now,  make  a  diagram 
of  a  pod  with  seeds,  and  by  a  broken  or  colored  line  show  the  path 
a  pollen-tube  must  take  from  the  style  to  an  ovule. 

The  Bean  Seed  and  its  Germination 

78.  Varieties  of  Beans.  —  (L)  Compare  color,  markings,  and  size 
of  specimens  of  some  of  the  common  varieties  of  beans  grown  in 
gardens.  (The  school-museum  should  have  a  collection  of  the  most 
common  varieties  of  beans  arranged  in  small  labeled  bottles  or  boxes.) 

The  variations  in  color,  size,  etc.,  of  the  seeds  are  no 
greater  than  the  variations  of  all  parts  of  the  plants  which 
grow  from  them  :  (1)  Bean  plants  may  be  low  (dwarf  beans), 
or  climbing  (e.g.,  lima  beans)  j  (2)  they  may  have  flowers  of 


82 


APPLIED  BIOLOGY 


various  colors;  (3)  the  leaves  may  differ  in  shape,  size,  and 
color;  (4)  the  pods  may  be  rounded  or  flattened,  short  or 
very  long  (in  one  variety  two  to  three  feet),  green  or  yellow 
(so-called  "wax  beans"),  with  "strings"  (§  77)  or  almost 
"  stringless,"  and  with  many  flavors;  (5)  some  bean  plants 
form  edible  pods  in  six  or  seven  weeks  after  planting  (the 
early  and  extra-early  varieties),  while  others  take  a  longer 
time  (lima  beans  are  often  killed  by  frost  before  the  seeds 

are  full  size). 
These  are  a  few 
of  the  variations 
of  beans  which 
interest  garden- 
ers, because  all 
these  are  quali- 
ties sometimes 
desired.  Look 
over  the  descrip- 
tions of  beans  in 
a  seed-catalogue, 
and  note  the 
points  empha- 
sized in  the  de- 
scriptions of  va- 
rieties offered  for 
sale. 

79.  Structure  of 
a  Bean.  —  (L)  Use 
any  large  beans 
(limas,  "yellow six- 
golden-eyed  wax"  are  excellent)  ; 
some  dry,  and  some  which  have  been  soaked  in  water  over  night. 
Examine  the  surface  markings.  Locate  the  scar  or  hilnm  and 
the  micropyle,  which  were  described  in  the  preceding  lesson  on  the 
pod.  The  translucent  elevation  seen  in  green  beans  near  the  hilum 
and  opposite  the  micropyle  is  colored  in  dry  beans  of  many  varieties 


FIG.  33.  Bean  seed  and  seedlings. 
e,  epicotyl;  h,  hypocotyl;  r,  roots, 
son.) 

weeks,"  "scarlet  runner,"  and 


c,  cotyledons; 
(From  Atkin- 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  83 

(dark  brown  in  "yellow  six-weeks,"  light  brown  in ' '  golden-eyed  wax  " 
beans).  These  conspicuous  marks  will  aid  in  locating  the  micropyle 
on  the  opposite  side  of  the  hilum.  Make  outline  drawings  of  a  bean, 
looking  down  upon  the  hilum,  and  also  as  seen  from  the  side. 

Strip  off  the  seed-coat  from  a  water-soaked  bean.  The  seed-coat 
has  two  layers,  which  are  easily  seen  in  a  green  bean.  The  main 
body  of  the  bean  consists  of  two  thickened  halves  (seed-leaves  or 
cotyledons).  These  are  stored  with  food  for  the  early  use  of  the 
young  plant  that  will  grow  from  the  seed. 

Carefully  separate  the  two  cotyledons,  and  notice  a  pointed  rod- 
like  body  which  is  joined  to  the  cotyledons.  This  is  called  hypocotyl 
(Fig.  33,  h).  It  is  best  seen  in  a  bean  which  has  begun  to  sprout,  for 
the  hypocotyl  then  pushes  through  the  seed-coat  at  the  micropyle. 
The  part  of  the  hypocotyl  next  the  cotyledons  will  form  the  begin- 
ning of  the  stem,  while  the  pointed  end  will  form  the  first  root.  Before 
sprouting  begins,  it  is  difficult  to  see  any  line  between  the  stem  and 
the  root  part  of  a  hypocotyl ;  and  we  simply  call  the  entire  structure 
hypocotyl  until  growth  makes  it  easy  to  distinguish  between  stem 
and  root.  By  placing  the  hypocotyl  in  certain  dyes  (e.g.,  solution 
of  permanganate  of  potash)  the  root  part  quickly  takes  up  the  stain 
and  makes  it  easy  to  see  that  the  pointed  end  (root)  of  the  hypocotyl 
is  different  in  its  cells  from  the  upper  or  stem  part. 

The  words  caulicle  and  radicle  which  are  used  in  some  botanical 
books  are  practically  synonymous  with  hypocotyl. 

Joined  to  the  hypocotyl  where  this  is  united  with  the  cotyledons 
is  a  very  short  stem  with  a  pair  of  small  leaves.  Between  these 
leaves  is  a  small  bud.  This  short  stem  with  leaves  and  bud  consti- 
tutes the  epicotyl.  It  will  form  the  stem  and  leaves  above  the 
cotyledons.  Some  books  call  the  bud  with  the  small  leaves  a  plumule, 
but  for  the  beginning  of  the  stem  and  leaves  above  the  cotyledons 
the  word  epicotyl  is  preferred. 

80.  The  Bean  Embryo.  —  Cotyledons,  hypocotyl,  and 
epicotyl  together  constitute  the  embryo,  which  may  de- 
velop into  a  bean  plant.  The  part  of  the  plant  which  de- 
velops from  each  part  of  the  embryo  is  as  follows :  — 

Cotyledons  —  not  very  useful  as  working  leaves 
of  bean  seedling,  but  stored  with  food. 


Bean  embryo 
consists  of 


Hypocotyl  —  forms  stem  below  cotyledons  and 
root  at  its  lower  end. 

Epicotyl  —  forms  stem  and  leaves  above  coty- 
ledons. 


84  APPLIED  BIOLOGY 

As  we  shall  see  in  later  studies,  the  seed-coat  is  lost  as  the 
embryo  develops;  it  is  therefore  simply  a  structure  for 
protecting  the  embryo  after  the  seed  is  out  of  the  pod  and 
until  germination  is  completed. 

81.  Germination  or  Awakening  of  Seeds.  —  A  dry  bean 
seed  shows  none  of  the  usual  signs  of  being  alive;  but  it 
soon  revives  when  placed  under  proper  conditions  (with 
moisture,  heat,  and  oxygen  from  the  air),  and  rapidly  grows 
into  a  young  plant  (called  seedling,  or  plantlet).  This 
"  awakening  "  or  reviving  and  growth  into  a  new  plant  is 
commonly  called  germination;  and  the  seed  is  said  to  ger- 
minate. The  popular  word  "  sprouting  "  usually  means  the 
early  stages  of  germination. 

Germination  of  Beans.  —  (L)  Beginning  about  two  weeks  before 
this  exercise,  some  beans  should  have  been  planted  every  other  day 
in  soil  (preferably  in  small  boxes  or  flower-pots  which  can  be  taken 
to  the  schoolroom).  Plant  about  two  inches  deep,  and  keep  the  soil 
moist  and  warm.  Plant  a  few  beans  four,  five,  and  six  inches  deep, 
and  some  near  the  soil  surface.  Mark  the  position  of  these  with 
wooden  stakes  on  which  figures  have  been  written  with  lead-pencil. 
When  some  .young  plants  (seedlings)  are  two  inches  above  the  soil, 
and  others  just  emerging,  the  materials  are  ready  for  the  following 
lesson. 

Without  pulling  up  any  plants,  carefully  examine  and  compare 
the  various  stages  in  order  to  determine  where  the  parts  of  the 
embryo  seen  in  the  seed  are  located  in  the  seedling.  What  becomes 
of  the  seed-coat  ?  The  first  joint  or  node  of  the  stem  is  where  the 
cotyledons  are  attached ;  the  second  at  the  leaves  of  the  epicotyl. 
Does  the  internode  between  these  two  nodes  lengthen?  Do  the 
cotyledons  of  the  bean  become  leaf -like  ?  Compare  the  size  and 
shape  of  the  leaves  of  the  epicotyl  with  those  you  have  seen  on  a  large 
bean  plant. 

Make  a  series  of  sketches  showing  stages  in  the  emergence  of  the 
seedling  from  the  ground.  Leave  space  in  note-book  for  adding 
other  and  larger  sketches  of  later  stages  as  the  plant  develops  for 
several  weeks. 

In  order  to  find  out  what  happens  between  the  time  the  seed  is 
planted  and  the  seedling  emerges  from  the  soil,  we  must  either  dig 
up  seeds  planted  at  various  periods  of  time  or  we  must  study  these 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


85 


early  stages  of  germination  in  seeds  which 
are  germinated  without  contact  with  soil. 
This  latter  is  best  because  the  particles  of 
•  soil  cling  to  the  seeds  and  make  them  more 
difficult  to  study. 

To  germinate  seeds  without  soil,  place 
them  between  layers  of  cotton  batting, 
sphagnum  moss,  sawdust  (some  kinds  are 
too  acid),  blotting-paper,  filter-paper,  or 
other  soft  papers.  Keep  moist,  not  wet,  and 
warm.  Or  simply  place  seeds  on  a  few 
layers  of  soft  paper  on  a  flat  dish  (such  as 
a  dinner-plate)  and  ke  p  moist,  warm,  and 
covered.  Sand  or  cotton  under  the  paper 
will  help  hold  moisture. 

(L)  Examine  various  stages  of  bean  seed- 
lings grown  without  soil.  Compare  care- 
fully with  seedlings  growing  with  their  roots 
in  soil,  and  make  sketches  of  the  chief  stages. 

(D)  In  order  to  determine  whether  any 
part  of  the  hypocotyl  grows  more  rapidly 
than  others,  make  equidistant  marks  with 
waterproof  India  ink,  and  observe  changes 
of  distances  between  marks  as  growth  pro- 
ceeds (see  Fig.  34). 


FIG.  34.  Hypocotyl  of 
bean  seedling  marked  to 
show  region  of  greatest 
growth  (between  0  and 
5).  (From  Strasburger.) 


THE  WORK  OF  THE  ORGANS  OF  A  PLANT :    AN  INTRO- 
DUCTION TO  PLANT  PHYSIOLOGY 

The  preceding  lessons  have  dealt  largely  with  the  structure 
of  the  various  parts  or  organs  of  the  bean  plant ;  and  all 
the  facts  to  which  special  attention  has  been  given  are  true 
of  the  vast  majority  of  the  plants  which  have  roots,  stems, 
and  leaves.  In  short,  the  bean  plant  has  been  studied  as 
a  type  of  the  plants  which  we  most  often  see  in  everyday 
life.  In  connection  with  the  study  of  structure,  some  brief 
mention  has  been  made  of  the  use  or  work  of  each  organ, 
but  fuller  explanations  have  been  reserved  for  this  section. 
As  far  as  possible,  the  following  physiology  lessons  are 
based  upon  the  bean  plant,  but  sometimes  we  shall  use 


86  APPLIED  BIOLOGY 

some  similar  plants  for  illustration  and  try  some  experiments 
with  plants  which  are  so  much  like  the  bean  plant  that  we 
have  good  reason  to  believe  that  the  work  of  their  organs  is 
the  same.  It  should  be  remembered,  then,  as  we  proceed 
with  the  study  of  plant  physiology  that  the  processes  or 
functions  being  studied  are  essentially  the  same  in  all  plants 
which  have  organs  such  as  we  have  found  in  the  bean  plant. 
Preview  of  plant  life.  —  In  order  to  carry  on  their  life- 
processes,  plants  must  have  food,  water,  and  oxygen,  as  ani- 
mals do.  Some  land  plants  get  the  first  two  of  these  essen- 
tials entirely  from  the  soil  and  others  in  part.  The  oxygen 
is  absorbed  chiefly  from  the  air  through  the  leaves.  Water 
from  the  soil  enters  the  roots  and  ascends  to  the  leaves, 
carrying  up  certain  food-materials.  In  green  plants  certain 
foods  are  formed  in  the  leaves  from  water  and  carbon  dioxide 
from  the  surrounding  air.  Sap  containing  foods  made  in 
the  upper  part  of  a  plant  may  flow  down  certain  tubes  in  the 
stem.  The  details  of  these  processes  will  now  be  considered. 

82.  The  Need  of  Water.  —  In  one  of  the  first  experiments 
we   found   that   plants  contain   a   large  amount  of  water. 
Moreover,  any  one  who  has  ever  cultivated  plants  knows 
that  unless  the  soil  is  kept  moist  the  plants  will  wither  and  die. 
Evidently  water  must  be  of  great  importance;    and  so  it 
will  be  of  interest  to  study  (1)  how  water  gets  into  the  plant 
organs  (root,  stem,  and  leaf)  and  (2)  what  work  water  does 
in  these  organs. 

83.  Source  of  Water.  —  It  is  obvious  that  ordinary  plants 
which  have  roots  must  get  most  of  their  water  from  the  soil. 
It  might  be  supposed  that  some  water  from  rain  and  dew 
which  wets  the  leaves  is  absorbed ;   but  that  this  is  exceed- 
ingly small  in  amount  and  insufficient  could  be  proved  by 
taking  a  potted  plant  and  covering  the  soil  with  waterproof 
cloth  so  that  rain  and  dew  touch  the  stem  and  leaves  but 
not  the  roots.     Under  such  conditions  most  kinds  of  ordinary 
plants  would  soon  wither. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  87 

84.  Water  in  Soil.  —  How  can  plants  get  water  from  soil 
which  appears  to  be  very  dry?     The  answer  to  this  question 
is  that  the  apparently  dry  soil  is  not  without  water.     It  is 
not  necessary,  or  even  desirable,  for  many  plants  that  the 
soil  be  wet ;    that  is,  contain  free  water  which  might  be 
drained  off.     On  the  contrary,  it  is  best  for  most  plants 
if  the  soil  contains  moisture  in  the  form  of  thin  films  ad- 
hering to  the  particles  of  the  soil.     We  have  already  noticed 
that  the  root-hairs  adhere  to  the  soil  particles  (Fig.  26),  and 
therefore  they  are  in  the  best  possible  position  for  absorbing 
the  moisture  of  the  soil.* 

85.  Rise  of  Water  in  Soil.  —  For  our  plant  studies  the  most 
important  points  concerning  soil  moisture  are  as  follows : 
The  water  stored  in  the  deeper  layers  of  the  soil  supplies 
moisture  to  the  layers  nearer  the  surface.     This  is  due  to 
capillary  attraction,  of  which  good  illustrations  are  the  rise  of 
oil  in  an  ordinary  lamp-wick  and  of  coffee  into  a  lump  of  sugar 
which  just  touches  the  liquid.     Water  is  continually  being 
lost  at  the  surface  of  the  soil,  owing  to  evaporation  and  to 
absorption  by  plants,  and   it  is   as   constantly  coming  up 
from  below.     But  it  may  be  lost  by  evaporation  from  the 
surface  soil  more  rapidly  than  it  can  come  from  the  deep 
layers,  especially  if  the  surface  is  hard-packed,  as  on  a  road, 
or  if  the  surface  is  covered  with  large  numbers  of  plants,  such 
as  weeds  or  grass,  which  take  much  water  from  the  soil. 

Mulching.  —  Excessive  loss  of  water  from  the  surface  of  the 
soil  may  be  prevented  by  mulching,  two  kinds  of  which  may 
be  illustrated  by  two  ways  of  raising  potatoes :  — 

(1)  In  the  usual  way  the  soil  is  cultivated  on  the  surface 
in  order  to  kill  weeds,  which  use  water  needed  by  the  growing 


*  If  the  students  working  with  this  book  have  not  in  some  previous  science 
work  had  lessons  on  water  in  soil,  it  is  suggested  that  some  work  in  this  line 
be  here  introduced.  Read  Osterhout's  "Experiments  with  Plants,"  pp. 
103-121;  and  Burkett,  Stevens  and  Hill's  "Agriculture  for  Beginners," 
pp.  10-15 ;  and  perform  the  experiments  suggested. 


88  APPLIED  BIOLOGY 

potato  plants,  and  also  in  order  to  keep  the  surface  of  the  soil 
in  a  dusty  condition.  This  checks  the  capillary  attraction 
a  few  inches  below  the  surface  and  moisture  comes  up  to 
within  reach  of  the  roots  of  potato  plants  but  not  to  the 
surface  where  it  will  be  wasted  by  evaporation.  Such  a 
condition  is  known  in  agriculture  as  a  dust-mulch.  On  many 
farms  which  are  conducted  according  to  modern  science  one 
may  see  on  hot  dry  days  in  the  summer  the  cultivating- 
machines  at  work  pulverizing  the  surface  of  the  soil  in  order 
to  keep  in  the  moisture.  This  is  the  main  secret  of  success 
in  the  "  dry  farming  "  in  many  western  states. 

(2)  The  second  kind  of  mulch  is  illustrated  by  the  following 
method,  often  successfully  used  in  growing  potatoes  in  dry 
regions  and  seasons.  The  potatoes  are  planted  shallower 
than  in  the  usual  method  and  the  entire  field  is  covered  with 
several  inches  of  straw.  The  potato  plants  grow  up  through 
the  straw,  but  most  of  the  ordinary  weeds  do  not.  The 
straw  prevents  evaporation  of  water  from  the  surface, 
and  in  dry  weather  the  soil  is  found  to  be  moister  than  soil 
treated  by  the  dust-mulch  method.  In  fact,  a  great  ob- 
jection to  the  method  is  that  the  soil  often  becomes  too  moist 
after  heavy  rains.  The  same  method  is  often  used  in  or- 
chards, cutting  the  weeds  and  grass  and  spreading  them  over 
the  ground  around  the  trees,  instead  of  cultivating  the  soil 
to  keep  in  the  moisture. 

It  is  obvious  from  the  foregoing  discussion  of  water  in 
soil  that  this  is  one  of  the  most  important  problems  connected 
with  growing  useful  plants,  and  horticulturists  and  agricul- 
turists have  found  it  important  to  understand  the  scientific 
facts  concerning  the  water  of  the  soil  and  its  use  by  plants. 

86.  Absorption  of  Water  by  Roots.  —  That  pressure  is 
developed  when  water  is  absorbed  by  the  roots  may  be 
demonstrated  as  follows  :  — 

(D)  Cut  the  stem  of  a  plant  near  the  ground,  and  attach  a  glass 
tube  by  means  of  rubber  tubing  as  shown  in  Fig.  35 ;  but  use  a 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


89 


straight  tube  and  put  in  a  small  quantity  of  water  instead  of 
mercury,  which  is  used  with  the  S-tube.  Geranium,  dahlia,  tomato, 
sunflower,  corn,  many  young  shrubs  and  trees,  and  grape-vines  have 
stems  which  make  it  easy  to  attach  the  tube.  Make  marks  on  the 
tube  and  note  the  rise  of  water  in  the  tube  for  several  days. 

Instead  of  a  straight  tube,  botanists  often  use  an  S-shaped 
tube,  and  fill  the  lower  bend  of  the  tube  with  mercury. 
Since  mercury  is  13.6  times  heavier  than 
water,  it  is  easy  to  compute  the  height  to 
which  a  column  of  water  might  be  forced. 
It  has  been  found  that  in  the  grape  the 
root-pressure  is  sufficient  to  force  a  column 
of  water  up  10  meters  (how  many  feet?). 

87.  Work    of    the    Root-Hairs.  —  How 
does  the  water  concerned  in  root-pressure 
get  into  the  root  from  the  soil?     This  is 
the  question  which  naturally  arises  in  our 
minds  when  we  observe  the  preceding  ex- 
periment.    Scientists  have  studied  the  mi- 
croscopic structure  of  the  roots  of  many 
plants  and  have  found  no  openings  or  pores   FlG 
through  which  water  can  get  into  the  root, 
and  so  they  have  concluded  that  the  water 
must  soak  through  or  be  absorbed  through 
the  walls  of  the  surface  cells  of  the  root  and 
especially  through  the  walls  of  root-hairs. 

In  order  to  make  this  method  of  absorption  clearer  we  must 
try  some  experiments. 

88.  Osmosis.  —  (D)    Select  a  cork  that  will  fit  firmly  in  the  mouth 
of  a  "diffusion  shell "  (a  membrane  bag  which  may  be  purchased  from 
dealers  in  scientific  apparatus),  bore  a  small  hole  in  the  cork,  and  into 
the  hole  fit  a  glass  tube  of  small  caliber  and  18  to  40  inches  long. 
Fill  the  shell  with  dark-colored  molasses,  insert  the  cork  in  the  shell, 
wrap  and  tie  around  the  cork  a  strong  coarse  thread,  and  support  with 
a  retort-stand,  wooden  tripod,  or  otherwise,  so  that  the  diffusion- 
shell  will  hang  in  water   (Fig.  36).     Note  the  movement  of  the 


tube 


(g,h)  with  mercury, 
attached  to  a  stem 
(s)  by  rubber  (r), 
to  measure  root- 
pressure.  (Det- 
mer.) 


90 


APPLIED  BIOLOGY 


molasses  up  the  tube  for  several  days.     Change  the  water  when  it 
becomes  discolored  by  exuded  molasses.     If    the    shell    does    not 
break,  or  leak  at  the  cork,  the  column  of  fluid  may  rise  to  a  height 
of  8  or  10  feet.     The  glass  tube  may  be  made 
longer  by  joining  on  other  tubes  with  pieces  of 
rubber  tubing  for  the  joints. 

Or  a  piece  of  gold-beater's  membrane,  or  of 
fish-bladder,  may  be  tied  over  the  funnel  of  a 
thistle-tube,  or  even  on  one  end  of  an  ordinary 
tube,  filled  with  molasses  in  the  same  way  and 
placed  in  water. 


The  above  experiment  shows  that  sugar 
solution  (molasses)  "  attracts "  water 
through  a  membrane  so  strongly  that  the 
pressure  developed  will  raise  a  column  of 
water  many  feet  in  height.  Like  roots, 
as  stated  above,  the  membrane  used  for 
the  experiment  has  no  visible  pores.  The 
water  must  pass  through  spaces  which  are 
far  too  small  to  be  seen  with  the  aid  of 
the  strongest  microscope.  Such  diffusion 
or  absorption  of  water  through  a  mem- 
brane without  visible  pores  is  called 
osmosis  (sometimes  osmose)  in  the  science 
of  physics.  The-  verb  to  osmose  will  be 
used  in  this  book.  For  an  explanation  of 
osmosis  one  must  study  the  advanced 


FIG.  36.  Diagram  of 
apparatus  for  os- 
mosis, t,  glass 
tumbler;  m,  mem- 
brane bag;  w,  level 
of  water  in  tumbler; 
s,  sugar-solution  in 
bag  and  up  to  level 
x\  c,  cork  tied  into 
mouth  of  the  bag; 
g,  glass  tube  of  ^ 
inch  bore  fitted  into 
hole  in  cork.  Rise 
of  fluid  higher  than 

x  indicates  osmosis    books  on  physics :  but  for  the  purposes 

of    water    into    the        ,     .  ,.,.«..  u 

sugar  solution  faster  of  plant  study  it  is  sufficient  to  remember 
than  that  of  the  the  above  experiment  showing  osmosis 
iato'the0wa?en  through  a  membrane  which  has  no  visible 

pores. 

Another  point  needed  for  our  later  studies  is  that  water 
having  substances  in  solution  may  osmose.  For  example, 
in  the  above  experiment  the  water  became  discolored  by 
the  exuded  molasses  (solution  of  sugar  in  water),  proving 


AN  INTRODUCTION   TO  PLANT  BIOLOGY  91 

that  some  of  the  molasses  osmosed.  Evidently  the  molasses 
did  not  osmose  outward  as  rapidly  as  the  water  osmosed 
into  the  molasses,  otherwise  the  column  of  fluid  would  not 
have  been  forced  up  the  tube. 

This  outward  osmosis  of  molasses  in  the  experiment  simply 
illustrates  the  fact  that  solutions  will  osmose  through  a  mem- 
brane ;  but  it  must  be  understood  that  sugar  solution  in  a 
plant  root  would  not  osmose  out  into  the  soil,  for  the  living 
plant  cell  can  prevent  outward  osmosis  of  its  constituent 
substances.  However,  the  osmosis  into  roots  of  substances 
dissolved  in  soil  water  might  be  illustrated  by  adding  common 
salt  to  the  water  outside  the  membrane  in  the  above  experi- 
ment, and  later  it  would  be  found  that  some  of  the  salt 
solution  had  osmosed  into  the  molasses.  A  large  number 
of  substances  in  solution  will  osmose  through  membranes, 
but  others  (e.g.,  white-of-egg,  and  glue)  will  not  osmose. 

89.  Osmosis  or  Absorption  by  Roots.  —  The  above  prin- 
ciple of  osmosis  applies  to  the  absorption  of  water  by  roots 
as  follows :  The  cells  of  the  roots  (especially  the  root-hairs, 
Fig.  26)  contain  cell-substances  which  attract  water  very 
much  as  the  molasses  did  in  our  experiment.  The  walls 
of  the  cells  allow  osmosis  as  the  membrane  of  the  diffusion- 
shell  did.  In  short,  water  from  the  soil,  containing  mineral 
substances  in  solution,  osmoses  into  the  cells  on  the  surface 
of  the  root,  especially  the  root-hairs.  There  is  one  dif- 
ference between  this  osmosis  in  the  root  and  that  in  our 
experiment ;  namely,  that  the  cells  of  the  root  are  filled 
with  substances  which  attract  water  but  do  not  themselves 
osmose  out,  as  did  the  molasses.  Hence  water  continually 
osmoses  into  the  root,  while  the  chief  cell-substances  do  not 
osmose  out  into  the  soil.  Some  substances  in  root  cells 
do  pass  into  the  soil,  but  in  very  small  quantities. 

Recalling  the  pressure  made  evident  by  the  height  of  the 
water  column  in  the  last  two  experiments  (§  88) ,  we  see  that  in 
both  cases  pressure  is  the  result  of  osmosis  or  absorption  of 


92  APPLIED  BIOLOGY 

water,  in  one  case  into  the  root  by  the  cell-substance,  and  in 
the  other  case  into  the  diffusion-shell  by  molasses.  Roots, 
then,  get  the  water  from  the  soil  by  a  process  called  absorption 
or  osmosis,  which  is  due  to  the  power  of  cell-substances 
to  "  attract  "  water  through  the  delicate  walls  of  the  root 
cells,  especially  of  the  root-hairs. 

90.  In  what  Part  of  the  Root  does  Water  ascend  to  the 
Stem  ?  —  Our  experiment  on  root-pressure  (§  86)  showed  that 
water  passes  from  the  soil  through  the  root  into  the  stem.     It 
has  been  stated  that  water  gets  into  the  root  through  its 
surface,  particularly  through  the  root-hairs,  which  greatly 
increase  the  amount  of  surface  cells  for  absorption.     What  is 
the  path  of  water  through  the  root  on  its  way  to  the  stem  ? 
The  following  experiment  gives  the  answer. 

(D)  Cut  off  the  small  end  of  a  slender  root  of  carrot  or  parsnip, 
or  any  other  plant  large  enough  for  convenience  in  cutting  sections ; 
place  cut  end  of  root  in  a  bottle  of  red  ink  (eosin  in  water) ;  after 
several  hours  cut  transversely  in  several  places,  and  note  the  posi- 
tion of  the  red-colored  water  in  the  tubes  in  the  woody  part  (in 
fibro- vascular  bundles,  §70).  Some  of  the  ink  after  a  time  soaks 
out  (osmoses)  into  the  cells  in  the  bark  and  pith  of  the  root.  Thus 
water  from  the  soil  is  distributed  to  cells  by  the  wood-tubes. 

91.  Why  Water  ascends  Stems.  —  There  are  many  evi- 
dences that  water  is  continually  ascending  the  stems  of 
plants  to  the  leaves.     For  example,  if  we  cut  the  stem  off  any- 
where between  the  root  and  the  leaves,  the  result  is  that 
the  leaves  soon  wither  and  dry  up,  while  similar  leaves  with 
the  natural  connection  to  the  root  through  the  stem  remain 
fresh  and  well  supplied  with  water.     We  have  already  studied 
the  rise  of  water  from  the  soil  into  the  root  and  from  the  root 
into  the  stem.     What  causes  the  water  to  ascend  the  stems 
of  plants?    This  is  still  one  of  the  unsolved  problems  of 
botany.     Root-pressure,  as  we  have  seen  (§  86),  is  enormous. 
In  a  young  birch  sapling  it  has  been  found  to  be  great  enough 
to  raise  a  column  of  water  18  meters  (how  many  feet  ?) .     But 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


93 


root-pressure  alone  is  not  enough  to  force  water  to  the  top 
of  tall  trees,  and  it  does  not  move  water  upward  fast  enough 
to  make  good  the  loss  by  evaporation  from  the  leaves  on  a 
hot  day.  There  must  be  other  factors  in  the  elevation  of 
water. 

Another  possible  explanation  is  suggested 
by  the  following  experiment :  — 

(D)  "Suction"  Force  of  Evaporation  from 
Leaves.  —  Cut  off  *  a  shoot  (stem  with  leaves)  of 
geranium  or  other  potted  plant  and,  with  a  short 
piece  of  rubber  tubing,  attach  to  a  glass  tube. 
Fill  the  tube  with  water  and  hold  under  water 
while  attaching  the  plant  stem.  See  Fig.  37. 
The  rubber  mus-t  be  firmly  wrapped  with  cord  so 
as  to  bind  it  to  the  shoot  and  to  the  glass.  It  is 
well  to  spread  a  little  vaseline  around  the  two 
ends  of  the  rubber.  Support  tube  with  a  retort- 
stand  so  that  the  lower  end  dips  into  mercury  in 
a  small  dish  or  bottle.  As  water  is  evaporated 
from  the  leaves,  the  mercury  will  rise  in  the  tube. 
If  mercury  is  lifted  one  inch,  it  indicates  a  force 
able  to  lift  water  13.6  inches.  In  this  way  it  has 
been  determined  that  some  shoots  will  lift  water 
several  meters.  Evidently  there  is  great  lifting 
power  or  suction  force  caused  by  evaporation 
from  the  shoot. 


FIG.  37.  Mercury 
(m)  lifted  in  glass 
tube  by  evapora- 
tion of  water  from 
leaves.  Stem 
closely  fitted  to 
tube  by  cork  at  c. 
(From  Detmer.) 


That  the  lifting  power  seen  in  the  leafy  shoot  is  due  merely 
to  evaporation  and  not  to  some  force  peculiar  to  living 
plants  may  be  demonstrated  by  tying  a  bladder  or  other  mem- 
brane over  a  thistle-tube  filled  with  water  and  suspended 
vertically  so  as  to  dip  into  a  cup  with  oil  or  mercury.  As 


*  For  this  and  similar  experiments  it  is  best  'to  bend  the  stem  so  as  to 
have  the  part  where  the  cut  is  made  under  water  at  the  moment  of  cutting. 
This  prevents  the  entrance  into  the  tubes  of  the  stem  of  air  bubbles  which 
impede  the  passage  of  water  just  as  they  can  be  seen  to  do  in  a  glass  tube 
of  small  bore.  Flowers  cut  in  this  way  will  not  wilt  as  soon  as  those  cut  off 
in  the  air.  (Cut  five  flowers,  or  leafy  shoots,  each  way,  stand  in  water  and 
compare  as  to  their  wilting  after  some  days.) 


94  APPLIED  BIOLOGY 

rapidly  as  the  water  evaporates  through  the  membrane,  the 
oil  or  mercury  rises  in  the  tube.  The  apparent  "  suction  "  is 
the  same  as  in  a  pump,  and  is  to  be  explained  by  atmospheric 
pressure  on  the  mercury  or  oil  forcing  it  up  the  tube  as  rapidly 
as  the  water  evaporates.  Possibly  the  same  explanation 
applies  to  the  evaporation  of  water  from  the  leaves  in  the 
above  experiment. 

It  has  been  stated  that  root-pressure  alone  does  not  ex- 
plain why  water  ascends  stems  of  high  plants,  because 
water  sometimes  evaporates  more  rapidly  than  root-pressure 
can  force  it  up.  The  last  experiments,  showing  that  evapora- 
tion of  water  may  exert  great  lifting  force,  suggest  to  us 
that  evaporation  from  the  leaves  of  plants  may  suck  or 
lift  water  up  the  stems.  This  is  certainly  one  important 
factor  in  causing  the  ascent  of  water  from  the  roots  to  the 
leaves;  but  evaporation  alone  could  not  lift  water  up  the 
highest  trees. 

Root-pressure  and  evaporation  from  the  leaves  are  the 
two  best  suggestions  concerning  the  rise  of  water  up  the  stems 
of  plants;  but  botanists  admit  that  these  together  do  not 
explain  how  a  plant  can  lift  water  as  high  as  do  the  highest 
trees.  This  is  only  one  of  many  hundreds  of  things  in  science 
which  it  has  not  been  possible  to  explain  satisfactorily; 
and  in  the  attempt  to  get  more  knowledge  hundreds  of  scien- 
tific men  are  constantly  trying  experiments  in  new  ways. 

92.  In  What  Part  of  Stem  does  Water  Ascend?  —  We 
have  already  examined  (§  69)  a  stem  of  bean  plant  and  found 
rind  or  bark,  wood,  and  pith.  Does  the  water  go  up  in 
all  these  regions  of  the  stem?  We  can  best  answer  this 
question  by  an  experiment. 

(D)  Cut  off  a  shoot  (stem  with  leaves)  of  a  nasturtium,  balsam, 
corn,  bean,  or  other  plant,  and  place  the  cut  end  of  the  stem  in  a 
small  bottle  with  red  ink  (solution  of  eosin  in  water).  Watch  the 
rise  of  the  ink  along  the  stem  into  the  veins  of  the  leaves.  Now 
cut  across  the  stem  in  various  places  and  note  that  the  red  color  is 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  95 

in  the  woody  part.  Cut  a  thin  section  and  note  that  the  red  color 
is  in  the  wood-tubes  of  the  nbro-vascular  bundles  which  were  men- 
tioned in  §  70. 

If  some  stems  be  left  in  the  ink  for  several  hours,  the  sections  will 
show  that  the  red  ink  soaks  (osmoses)  from  the  wood-tubes  into  the 
bark  and  pith. 

From  such  experiments  as  those  above  we  conclude  that 
the  path  of  water  up  the  stem  is  through  the  nbro-vascular 
bundles  in  the  woody  part  of  the  stem.  It  was  through 
these  tubes  that  the  water  passed  in  our  experiment  on  root- 
pressure  (see  §86)  and  in  that  on  evaporation  (§  91).  From 
the  wood-tubes  water  slowly  osmoses  into  the  cells  of  the 
bark  and  pith. 

93.  Water  in  the  Leaf,  and  Evaporation.  —  In  the  last 
experiment  the  red-colored  water  ascended  the  stem  and 
passed  along  the  veins  of  the  leaf.  This  was  possible  be- 
cause the  veins  of  the  leaf  are  bundles  of  tubes  directly 
connected  with  the  wood-tubes  in  the  stem. 

(D)  Take  a  leaf  from  the  bean  or  other  shoot  which  was  used  in 
§  92.  Scrape  the  petiole  and  veins  so  as  to  show  the  bundles  of 
tubes  colored  by  the  red  ink.  Note  (especially  good  in  the  bean 
and  in  celery)  the  connection  of  the  bundles  of  tubes  in  the  veins  of 
the  leaf  with  the  bundles  of  the  stem ;  this  is  easily  done  by  care- 
fully scraping  away  the  surface  tissue  from  one  side  of  the  stem  and 
petiole  until  the  colored  bundles  of  tubes  are  uncovered. 

It  is  evident  from  the  arrangement  of  the  tubes  and  the 
path  taken  by  the  ink  that  water  can  pass  directly  from  the 
wood-tubes  of  the  stem  into  those  of  the  veins  of  the  leaf,  and 
thence  into  the  numerous  veinlets.  In  this  way  water 
coming  up  the  stem  from  the  root  is  distributed  throughout 
the  leaf,  which  is  thin  and  greatly  expanded  so  as  to  expose 
as  much  surface  as  possible  to  sunlight  and  air,  and  thus  pro- 
mote rapid  evaporation.  It  is  obvious  that  one  purpose  of 
the  veins  of  the  leaf  is  to  distribute  or  spread  the  water  so 
that  it  may  be  evaporated  rapidly.  That  the  arrangement  is 
very  efficient  is  indicated  by.  the  fact  that  the  leaves  of  a 


96  APPLIED  BIOLOGY 

sunflower  plant  six  feet  high"  have  been  proved  to  evaporate 
a  liter  (how  many  pints  ?)  of  water  per  day ;  and  it  has  been 
carefully  estimated  that  a  large  birch  tree  with  about  200,000 
leaves  gave  off  500  liters  of  water  (2J  barrels)  on  a  dry, 
hot  day  and  probably  averaged  60  to  70  liters  per  day  during 
the  summer. 

Estimating  Loss  of  Water  from  a  Plant.  —  (D)  Take  a  potted 
geranium,  or  other  convenient  plant,  surround  the  pot  and  cover 
the  soil  with  a  sheet  of  rubber,  tin-foil,  or  waterproof  cloth,  and 
tie  carefully  around  the  stem  of  the  plant.  Or  set  the  pot  in  a 
battery-jar  and  cover  the  top  with  sheet  rubber  tied  tightly  around 
both  jar  and  stem  of  plant.  Thus  only  the  upper  stem  and  leaves 
will  be  exposed  for  evaporation.  Now  place  the  potted  plant  on 
a  small  platform  scale  (preferably  one  with  two  equal  platforms 
which  balance  each  other),  and  add  the  weights  until  there  is  an 
exact  balance.  Note  from  day  to  day  the  loss  of  weight  from  the 
plant.  A  glass  funnel  may  be  inserted  through  the  waterproof 
covering  into  the  soil,  and  a)weighed  amount  of  water  [poured  in 
daily  to  replace  the  amount  lost  by  evaporation. 

In  order  to  make  evaporation  more  rapid  than  the  slow 
drying  from  the  surface  cells  of  the  leaf,  the  leaf-pores 
(stomata)  are  sometimes  opened  (as  described  in  §73),  al- 
lowing watery  vapor  to  escape  from  air-spaces  in  the  leaf 
(Fig  30).  These  cavities  are  surrounded  by  cells  from 
which  evaporation  of  water  takes  place  rapidly.  Thus  the 
giving  off  of  water  is  more  rapid  when  the  leaf-pores  are 
open,  and  less  when  they  are  closed.  The  leaves  of  many  kinds 
of  plants  seem  to  have  a  very  complete  control  of  the  amount 
of  water  evaporated,  because  in  these  species  the  covering 
cells  of  the  leaves  are  thick  and  sometimes  coated  with 
waxy  substances  or  hairs,  and  in  still  other  ways  are  un- 
favorable for  the  evaporation  of  water  from  the  surface  of  the 
leaf.  In  such  plants  evaporation  appears  to  take  place 
chiefly  through  the  leaf -pores.  When  we  remember  that  an 
ordinary  cabbage  leaf  has  ten  million  and  a  sunflower  leaf 
thirteen  million  leaf-pores,  we  can  easily  understand  how 


AN  INTRODUCTION  TO  PLANT  BIOLOGY  97 

the  opening   and   closing  of  these   pores  may  control  the 
amount  of  evaporation. 

The  process  of  evaporation  of  water  from  the  leaves  of 
plants  is  in  botany  commonly  called  transpiration,  and  we 
say  that  leaves  transpire,  meaning  that  they  give  off  water 
by  evaporation.  For  all  practical  purposes  the  words 
evaporation  and  transpiration  are  equivalent  as  applied 
to  the  work  of  leaves. 

94.  Soil- Water   and    Sap.  —  Throughout   this   lesson  we 
have  referred  to  water  as  ascending  in  the  plant,  but  it  is 
never  pure  water.     Water   absorbed   from   the   soil   always 
contains    some    mineral    materials    in    solution.     Also,    as 
the  water  passes  through  the  plant  it  absorbs  or  dissolves 
other  substances  and  becomes  sap.     However,  a  large  part 
of  the  water  which  goes  directly  up  the  stem  in  the  wood- 
tubes  is  not  very  different  from  the  water  of  the  soil ;   and 
travelers  in  tropical  countries  often  cut  off  certain  vines 
and  drink  the  water  which  runs  from  the  stems. 

One  other  point  should  be  emphasized;  namely,  that  in 
the  evaporation  of  water  there  is  left  behind  in  the  plant, 
especially  in  the  leaves,  all  the  substances  brought  in  solu- 
tion from  the  soil.  Leaves  which  fall  in  the  autumn  give  by 
burning  much  more  ashes  than  do  leaves  which  fall  in  the 
early  summer;  and  the  explanation  is  that  evaporation 
during  the  long  summer  season  has  left  behind  in  the  leaf 
various  materials  carried  up  in  solution  in  water  from  the 
soil,  and  not  needed  by  the  plant. 

95.  Use  of   Water  in  the  Plant.  —  So  far  in  our  study 
of  water  and  its  movement  through  root,  stem,  and  leaves, 
and  its  final  loss  through  evaporation,  we  have  considered 
the  purely  mechanical  processes  and  without  reference  to 
the  use  of  the  water  while  passing  through  the  plant.     Now, 
plants  are  not  elevating  water  simply  for  the  sake  of  evaporat- 
ing it  from  the  leaves  ;  on  the  contrary,  evaporation  is  neces- 
sary to  make  place  for  more  fresh  water  and  so  keep  up  the 


98  APPLIED  BIOLOGY 

current  from  root  to  leaves.  This  is  necessary  for  the  follow- 
ing reasons :  (1)  The  water  carries  up  in  solution  indispen- 
sable food  materials  obtained  from  the  soil.  (2)  Water  is 
necessary  in  many  parts  of  the  plant  in  order  to  give  turgidity 
and  rigidity.  Without  plenty  of  water,  the  plant  wilts, 
which  is  due  to  lack  of  water  in  cells  and  consequent  loss 
of  turgidity.  A  Windsor  bean  plant  grown  in  a  pot  is  excel- 
lent for  showing  this.  Allow  the  soil  to  dry  until  the  plant 
wilts,  then  water  the  soil.  (3)  Water  is  necessary  in  order  to 
dissolve  sugar  (e.g.,  maple  sap  and  other  sweet  juices  of 
plants)  and  other  food-substances  which  must  often  be 
transported  in  solution  from  one  part  of  a  plant  to  another. 
(4)  Water  is  needed  because  it  is  used  in  the  chemical  com- 
bination of  such  food-substances  as  starch,  sugar,  and  oils 
which  are  made  by  the  cells  of  the  plant.  (5)  Water  in  large 
quantities  is  needed  by  growing  plants  because  such  a 
large  proportion  of  the  substance  of  new  cells  is  water. 
(6)  Evaporation  of  water  results  in  cooling  the  plant,  thus 
preventing  a  dangerous  amount  of  internal  heat. 

In  addition  to  these  special  reasons  why  plants  need  water, 
we  must  remember  that  all  living  matter  requires  water 
(§  12).  Without  water  there  is  no  life,  so  far  as  we  know. 
Even  seeds  which  are  apparently  dry  contain  a  certain  amount 
of  water  (8  to  15  per  cent  of  their  weight). 

96.  Food  Requirements  of  Plants.  —  In  one  of  the  first 
lessons  (§  16)  we  found  that  a  plant  is  made  up  of  water, 
carbon,  and  mineral  matters  (in  ashes),  and  also  some  carbon 
and  other  elements  making  up  the  gases  which  were  burned. 
When  chemists  carefully  analyze  these  substances  from 
plants  they  find  the  following  ten  chemical  elements  :  carbon, 
(C),  hydrogen  (H),  oxygen  (O),  nitrogen  (N),  sulphur  (S), 
phosphorus  (P),  iron  (Fe),  potassium  (K),  calcium  (Ca), 
and  magnesium  (Mg)  in  every  plant ;  and  still  other  elements 
are  found  in  many  plants,  but  are  not  absolutely  necessary 
for  plant  life.  The  first  four  (C,  H,  O,  N)  form  the  chief  part 


AN  INTRODUCTION   TO  PLANT  BIOLOGY  99 

of  the  combustible  matter  of  all  plants.  These  ten  elements 
which  are  always  found  in  plants  must  also  be  in  their  food, 
and  in  the  next  three  sections  we  shall  consider  how  plants 
get  the  food  which  will  furnish  these  necessary  elements  for 
making  plant  structure. 

97.  Food-Materials  from    the  Soil.  —  It  is  a  matter  of 
common  observation  that  the  growth  of  plants  is  largely 
influenced  by  the  nature  of  the  soil.     Every  farmer  and 
gardener  learns  through  experience  to  distinguish  between 
barren  and  fertile  soils,  and  that  the  addition  of  manures 
and  various  chemical  "  fertilizers  "  increases  the  growth  of 
plants.     The    relation    of   plant    growth    to    the    materials 
available  in  the  soil  may  be  well  illustrated  by  the  following 
experiment. 

(D)  Growing  Plants  without  Soil.  —  This  may  be  done  by  ger- 
minating seeds  of  oats,  beans,  peas,  lupines,  and  other  common 
plants  on  moist  sawdust,  cotton,  sand,  crushed  stone,  or  other 
materials  into  which  roots  can  penetrate,  but  which  contain  no  plant 
food.  When  the  seeds  are  well  germinated,  begin  to  moisten  the 
sawdust  or  cotton  daily  with  water  in  which  has  been  dissolved  some 
chemical  tablets  containing  the  materials  such  as  are  found  in 
good  garden  soil;  that  is,  the  necessary  elements  (§  96).  If  the 
roots  are  kept  moist  with  such  a  solution  of  chemicals,  some  plants 
will  develop  flowers  and  seeds.  By  trying  various  chemicals  in 
such  experiments,  it  has  been  possible  for  botanists  to  prove  that 
only  certain  elements  are  necessary  in  the  soil  for  plant  growth. 
Most  of  the  elements  are  common  in  agricultural  soils,  but  com- 
mercial fertilizers  rich  in  nitrogen,  potassium,  calcium,  and  phos- 
phorus are  needed  on  many  farms.  The  tablets  may  be  purchased 
from  the  Agassiz  Association,  Sound  Beach,  Conn.,  for  ten  cents  a 
box,  post  paid. 

Other  interesting  experiments  in  the  same  line  may  be  performed 
by  growing  plants  in  different  kinds  of  soil  in  pots,  fertilizing  the  soil 
with  various  kinds  of  plant  foods  and  fertilizers  sold  for  garden  use 
(see  catalogues  of  seed-dealers). 

98.  Food  of  Plants  without  Chlorophyll.  —  We  can  better 
understand  how  a  bean  plant  or  other  green  plant  gets  its 


100  APPLIED  BIOLOGY 

food  if  we  first  study  the  nutrition  of  plants  like  the  mush- 
rooms and  the  Indian  pipe  (Monotropa),  which  have  no 
chlorophyll.  Such  plants  get  their  food-materials  entirely 
from  the  soil  in  the  form  of  (1)  certain  inorganr&  or  mineral 
substances  which  are  commonly  found  dissolved  in  water 
in  good  soils,  and  (2)  organic  materials  absorbed  from  the 
decomposing  matter,  such  as  the  leaf-mold,  on  which  mush- 
rooms and  the  Indian  pipe  commonly  grow. 

The  water  obtained  from  the  soil  by  mushrooms  contains 
compounds  with  the  elements  nitrogen,  and  also  in  various 
combinations  are  other  elements  (e.g.,  calcium,  phosphorus, 
sulphur,  potassium,  magnesium,  iron)  which  chemists  find  in 
analyzing  such  plants.  These  necessary  elements  may  come 
in  part  from  the  decaying  organic  materials  in  the  soil. 

It  is  from  this  decaying  matter  only  that  the  mushroom 
can  get  the  indispensable  foods  known  as  carbohydrates,  a 
term  which  means  a  compound  containing  carbon  (C)  and 
water  (H20),  and  therefore  composed  of  the  necessary  ele- 
ments carbon,  hydrogen,  and  oxygen  (C,  H,  O).  Examples 
of  carbohydrates  are  starch,  sugar,  and  cellulose  (the  chief 
substance  in  plant  cell-walls). 

Now,  mushrooms  and  other  plants  without  chlorophyll 
cannot  make  such  foods  as  starch  and  sugar,  and  so  they 
must  use  these  foods  made  by  some  pre-existing  plant  which 
had  chlorophyll ;  that  is,  they  must  live  on  leaf-mold  or 
other  decaying  plant  matter  from  which  they  can  absorb 
carbohydrate  food  in  a  dissolved  form,  usually  as  sugar. 
These  foods  which  are  absorbed  by  the  cells  of  the  mush- 
room are  used  by  its  protoplasm  in  the  life-activities  of  the 
cells,  especially  in  making  new  cell-materials  into  which  the 
nitrogen  and  the  other  necessary  elements  named  above  are 
also  combined. 

Saprophytes  and  Parasites.  —  Plants  which  get  their 
carbohydrate  foods  from  decaying  organic  matter  are  often 
called  saprophytes  (from  Greek  words  for  rotten  and  plant). 


AN  INTRODUCTION  TO  FLAX1    BIQL'IG*-         101 


Most  plants  without  chlorophyll  are  saprophytes  ;  but  some 
of  them  absorb  the  necessary  food  from  living  plants  or 
animals,  and  such  plants  are  called  parasites  (e.g.,  dodder, 
and  mistletoe  to  some  extent). 

99.  Food  of  Plants  with  Chlorophyll.  —  Such  plants  have 
in  their  bodies  at  least  the  same  ten  essential  elements  (§  96) 
as  plants  without  chlorophyll.  Nine  of  these  elements  (all 
except  carbon,  and  some  oxygen)  are  obtained  from  the  soil, 
the  hydrogen  and  oxygen  in  the  form  of  water  (H20)  and 
the  other  seven  in  solution  in  water  absorbed  by  the  roots. 
The  one  fundamental  difference  between  the  nutrition  of 
plants  with  and  without  chlorophyll  is  that  the  chlorophyll  is 
a  special  substance  with  the  aid  of  which  the  protoplasm  of 
leaf-cells  is  able  under  the  action  of  light  to  make  the  car- 
bohydrate food  needed  by  the  plant,  while  plants  without 
chlorophyll  must  absorb,  by  their  roots,  such  food  from 
the  decaying  bodies  of  other  plants  or  animals. 

Cells  containing  chlorophyll  are  able  in  light  to  make  car- 
bohydrate food  in  the  form  of  starch  or  sugar,  obtaining  the 
necessary  elements  from  water,  from  the  soil,  and  from  carbon 
dioxide  from  the  air.*  This  production  of  sugar  and  starch 
takes  place  chiefly  in  the  leaves.  Probably  sugar  is  first 
formed  and  then  changed  into  starch  ;  but  since  starch  is 
usually  demonstrable  in  green  leaves  exposed  to  light,  we 
shall  give  special  attention  to  its  formation.  However,  it 
makes  no  difference  to  the  plant  whether  sugar  or  starch 
is  formed  in  the  leaves,  for  they  are  of  equal  value  as  food 
for  all  plant  cells. 

The  stomata  allow  watery  vapor  to  escape,  in  transpiration, 
and  they  are  also  important  in  allowing  air  with  carbon  di- 


*  The  atmosphere  contains  on  the  average  about  three  parts  of  carbon 
dioxide  in  10,000  of  air,  measured  by  volume ;  and  yet  from  this  exceedingly 
small  amount  of  carbon  dioxide  the  green  plants  get  the  necessary  carbon  for 
making  all  carbohydrates,  which  compose  a  large  part  of  the  solid  matter 
of  plants. 


102  APPLIED  BIOLOGY 

oxide  to  enter  the  spaces  of  the  leaves,  whence  the  carbon 
dioxide  passes  into  the  cells  with  chlorophyll  (Fig.  30). 
Water  reaches  these  cells  by  osmosing  from  near-by  veinlets, 
which  receive  water  from  the  roots  through  the  stem. 

100.  Photosynthesis.  —  That  light  is  necessary  for 
carbohydrate-making  (sugar  and  starch),  the  following 
experiment  shows.  Since  the  process  is  a  synthesis  or  a 
combining  depending  upon  the  action  of  light,  it  is  commonly 
termed  photosynthesis  (meaning,  combining  by  light) .  Briefly, 
it  is  starch-making  or  sugar-making  in  green  leaves  exposed 
to  light. 

Test  for  Starch.  —  (D)  Put  a  small  quantity  of  corn  starch  in  a 
tube  with  water,  add  a  few  drops  of  iodine  solution  (crystals  of  I  in 
70  per  cent  alcohol).  Note  the  blue  color.  Try  effect  of  iodine  on 
powdered  sugar  in  water.  If  we  had  time,  we  might  test  in  the  same 
way  all  the  various  substances  found  in  plants  and  animals ; 
but  we  must  accept  the  records  of  science  that  no  one  has  yet  found 
any  other  substance  which  with  iodine  solution  gives  this  peculiar 
blue  color  seen  in  starch.  Hence  we  use  iodine  as  a  test  for  the 
presence  of  starch  in  animal  and  plant  substances.  Sometimes 
iodine  is  mixed  with  chloral  hydrate  in  order  to  make  it  stain 
more. 

Starch  Formed  in  Leaves  in  Light.  —  (D)  Take  two  potted  plants 
(bean,  nasturtium,  or  other  convenient  plants),  leave  in  a  perfectly 
dark  room  (or  box)  one  day,  and  on  the  morning  of  the  second 
day  take  one  of  the  plants  from  the  dark  and  set  in  strong  sunlight. 
Near  the  close  of  the  day  take  leaves  from  both  plants,  dip  into 
boiling  water  for  a  minute,  and  place  in  bottles  (labelled  "dark," 
"light")  containing  strong  alcohol.  When  the  class  meets  again, 
note  that  the  green  color  (chlorophyll)  has  been  extracted  by  the 
alcohol  ("bleached").  Take  a  leaf  from  each  bottle,  rinse  in  water, 
and  place  in  a  beaker  or  a  small  saucer  containing  some  iodine  solu- 
tion (or  better,  use  saturated  solution  of  chloral  hydrate  mixed  with 
enough  iodine  solution  to  color).  In  which  leaf  does  the  iodine 
test  show  presence  of  starch?  Remembering  that  the  two  leaves 
have  been  treated  exactly  alike  except  that  one  came  from  a  plant 
left  all  day  in  the  sunlight,  while  the  other  remained  in  the  dark, 
state  your  conclusion  as  to  the  importance  of  sunlight  in  starch- 
formation. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


103 


Another  way  of  showing  the  effect  of  light  on  starch-formation 
is  as  follows :  Take  a  piece  of  sheet  cork,  or  thick  pasteboard,  or 
tin-foil,  about  two  inches  square,  and  cut  out  a  triangle,  star,  or  any 
figure  preferred,  from  the  center.  By  means  of  pins  or  paper  clamps 
fasten  this  on  the  upper  side  of  a  leaf  of  a  potted  plant,  and  a  piece 
of  black  cloth  or  sheet  of  cork  on  the  lower  side ;  or  cut  the  opening 
desired  in  a  strip  of  tin-foil  and  then  fold  this 
around  the  middle  of  the  leaf.  Set  the  plant  in 
bright  sunlight  with  this  leaf  supported  so  that  the 
upper  side  will  get  full  illumination.  Near  the 
close  of  the  day,  cut  off  the  leaf,  dip  into  boiling 
water,  then  place  in  alcohol  for  a  day  or  two. 
Finally,  test  for  starch  with  the  iodine  (or  chloral 
hydrate  and  iodine)  as  in  above  experiment. 
Make  a  sketch  of  the  leaf,  and  by  colored  pencil 
or  shading  indicate  where  starch  was  formed. 
Label  the  portions  of  the  leaf  '*  exposed  to  light " 
and  "in  dark.'! 


FIG.  38.  The  darker 
ends  of  this  leaf 
were  colored  by 
iodine,  indicating 
starch.  The 
center  was  pro- 
tected from  sun- 
light. 


Variegated  Leaves.  —  Many  varieties  of 
plants  grown  in  greenhouses  have  "  varie- 
gated "  leaves  with  large  white  areas  due  to 
the  absence  of  chlorophyll.  That  starch  is 
not  formed  in  these  areas  can  be  shown  by 
taking  a  leaf  which  has  been  in  sunlight  all 
day  and  testing  for  starch  as  in  the  above 
experiments.  Plants  grown  for  a  long  time 
in  a  dark  room  lose  their  chlorophyll  (e.g.,  sprouting  potatoes 
in  a  cellar),  and  after  a  day  in  sunlight  their  leaves  show 
no  starch. 

Intensity  of  Light  Required.  —  It  must  not  be  inferred  from 
the  preceding  experiments  that  light  must  necessarily  be 
in  the  form  of  bright  and  direct  sunlight.  Starch-formation 
is  most  rapid  in  direct  sunlight,  but  probably  goes  on  in 
light  of  all  intensities,  even  in  moonlight.  This  explains 
why  some  species  of  plants  can  live  and  grow  slowly  in 
shaded  spots  in  the  woods;  they  apparently  make  use  of 
the  diffuse  and  weak  light  which  reaches  them.  Moreover, 


104  APPLIED  BIOLOGY 

light  from  other  sources  than  the  sun  will  serve  for  photo- 
synthesis; for  example,  electric  lights  are  sometimes  used 
in  greenhouses.  The  blooming  of  Easter  lilies  may  be 
hastened  from  four  to  ten  days;  and  lettuce  grown  within 
fifty  or  sixty  feet  of  a  2000-candle-power  arc-light,  used 
regularly  half  of  the  night,  will  be  ready  for  market  a  week 
or  more  before  plants  not  so  treated.  Incandescent  electric- 
and  gas-lights  have  a  similar  effect,  but  in  lesser  degree. 

Another  illustration  of  the  effect  of  light  is  in  the  rapid 
maturing  of  plants  under  the  influence  of  the  intense 
light  of  the  short  arctic  summer.  Also,  plants  in  greenhouses 
in  winter  do  not  grow  as  rapidly  in  a  given  number  of  hours 
of  sunshine  as  they  do  in  summer  when  the  light  is  so  much 
more  intense.  And  this  is  so  even  when  the  interior  of  a 
greenhouse  is  constantly  at  the  average  summer  heat. 

101.  Disappearance  of  Starch  from  Leaves.  —  If  we  take 
any  plant  which  has  been  standing  in  sunlight,  clip  off 
some  leaves,  and  place  in  alcohol  for  later  testing  to  make  sure 
that  starch  is  present,  and  then  set  the  plant  in  a  dark 
room  or  box  for  a  night  and  take  other  leaves  before  light 
reaches  them,  there  will  be  no  blue  color  with  iodine  solu- 
tion, thus  indicating  that  starch  has  not  remained  in  the  leaves 
kept  in  darkness.  Where  has  it  gone  during  the  night? 
There  are  two  answers  :  — 

First,  some  of  the  starch  has  probably  been  used  in  the  cells 
of  the  leaf,  either  changing  to  other  cell-substances  composed 
of  the  same  elements  (e.g.,  sugar),  or  combining  with  the 
elements  brought  in  water  from  the  soil  and  forming  com- 
pounds containing,  besides  the  C,  H,  O  of  the  starch,  some 
N,  S,  and  P,  and  perhaps  other  essential  elements.  The 
compounds  thus  formed  (containing  C,  H,  O,  N,  S,  P)  are 
called  albuminous  substances  or  proteins;  and  some  of  these 
may  form  some  new  living  matter  (protoplasm)  in  the  cells 
of  the  leaf. 

The  second  explanation  of  the  fact  that  the  starch  dis- 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         105 

appears  from  the  leaf  at  night  is  that  it  goes  down  into 
the  stem  or  root,  or  into  the  developing  flowers  or  fruits. 
That  this  is  what  occurs  in  part  is  proved  by  the  fact  that 
starch  and  other  foods  (e.g.,  sugar,  with  the  same  elements) 
are  found  in  these  parts  of  the  plant,  while  all  experiments 
ever  tried  have  failed  to  show  that  these  substances  can  be 
formed  in  cells  without  chlorophyll.  The  carbohydrate 
foods  needed  in  all  living  cells  of  a  plant  must  come  from  the 
cells  with  chlorophyll,  most  of  which  are  in  leaves. 

Microscopic  study  shows  that  starch  is  in  the  form  of 
grains  inside  of  cells  in  leaves. 

Starch  in  Cells.  —  (D)  Thin  sections  of  a  potato  will  show  the 
starch-grains  in  the  cells.  Stain  some  sections  with  iodine  solution 
before  mounting,  or  by  drawing  iodine  under  the  cover-glass  with 
blotting-paper.  Note  the  form  of  the  starch-grains. 

102.  Digestion  of  Starch  to  Sugar.  —  How  can  such  solid 
grains  get  through  the  cell-walls  in  going  from  the  leaves 
and  then  into  cells  in  other  organs?  The  answer  to  this 
question  is  that  starch  is  easily  changed  or  digested  to  sugar, 
which  is  soluble  in  water  and  can  osmose  from  cell  to  cell. 
This  change  from  starch  to  sugar  is  caused  by  a  substance 
known  as  diastase,  which  is  present  in  cells  of  the  leaves 
and  other  organs  of  plants.  This  substance  may  digest 
starch  at  all  times  during  the  day  and  night,  but  the  starch  is 
formed  during  sunlight  more  rapidly  than  it  can  be  digested. 
At  night  when  starch  is  not  being  formed  the  diastase  succeeds 
in  digesting  all  the  starch  which  was  left  in  the  leaf  at  the 
close  of  daylight.  Hence,  starch-grains  are  not  found  in 
leaves  in  the  morning  after  being  in  darkness  for  several 
hours;  but  sometimes  it  is  possible  by  chemical  analysis 
to  find  in  leaf  cells  sugar  into  which  the  starch  has  been 
changed  by  enzymes,  and  which  either  will  be  used  in  the  leaf 
or  will  pass  down  the  petiole  into  the  stem. 

Sugar  Test.  —  (D)  Boil  a  few  grapes,  raisins,  or  prunes  in  a  test- 
tube  with  a  little  water  in  order  to  extract  some  of  their  sugar. 


106  APPLIED  BIOLOGY 

Pour  some  of  the  extract  into  another  tube,  add  a  few  drops  of 
Fehling's  solution  (a  mixture  of  copper  sulphate  and  Rochelle  salts, 
used  by  chemists  for  testing  certain  kinds  of  sugar),  heat  tube  in  a 
flame,  and  note  the  red  color  of  the  contents.  Test  some  glucose 
or  corn-sirup.  Try  Fehling's  solution  on  a  little  starch  in  water ; 
does  the  red  color  appear?  Only  sugars  like  glucose  give  the  red 
reaction.  White  granulated  sugar  from  cane  and  beet  does  not. 

Change  of  Starch  to  Sugar.  —  (D)  Boil  a  small  quantity  of  starch 
in  water  in  a  test-tube,  thus  making  a  very  thin  starch  paste.  Put 
half  of  the  paste  in  a  second  test-tube  and  add  some  diastase  (ob- 
tained by  extraction  from  plant  tissues,  and  sold  at  drug-stores). 
After  a  half-hour,  take  some  liquid  from  each  tube  and  apply  the 
starch  and  sugar  tests.  Results  ?  Conclusions  ? 

To  test  regarding  osmosis  :  pour  the  contents  of  the  two  test-tubes 
into  two  gold-beater's  bags  and  hang  the  bags  in  tumblers  or  beakers 
containing  some  water,  or  use  the  osmose-apparatus  described  in 
§  398.  After  allowing  an  hour  or  more  for  osmosis,  pour  some  water 
from  each  tumbler  into  test-tubes,  and  test  with  a  few  drops  of 
Fehling's  solution.  Also  test  some  of  the  water  for  starch,  using 
iodine.  Does  the  starch  paste  osmose  ?  Is  there  sugar  in  the 
water  having  the  starch  without  diastase?  Conclusion? 

103.  Path  of  Foods  Down  the  Stem.  —  It  has  been  stated 
above  that  the  foods  (chiefly  sugar)  derived  from  the  starch 
of  the  leaf  may  go  down  into  the  stem  or  into  other  parts  of 
the  plant  connected  with  the  stem.  Also,  the  proteins  (com- 
pounds of  C,  H,  O,  with  N  and  other  elements)  which  are 
mentioned  in  §  101  as  being  formed  in  the  cells  of  leaves,  may 
go  down  into  the  stem  and  thence  into  roots,  flowers,  fruits,  or 
new  branches.  Obviously  the  sugar  and  proteins  in  solution 
cannot  go  down  in  the  wood-tubes,  because  in  them  there 
is  the  strong  upward  current  produced  by  transpiration. 
Botanists  are  now  agreed  that  the  downward  current  is  in 
the  bark  part  of  the  fibro-vascular  bundles,  and  through  the 
tubes  previously  described  (§  70)  as  sieve-tubes.  The  move- 
ment is  so  slow  that  it  cannot  be  demonstrated  with  red 
ink,  as  in  the  case  of  the  wood-tubes  (§  92).  However,  the 
evidence  obtained  from  other  experiments  is  no  less  con- 
vincing. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         107 

Effect  of  Girdling.  —  If  a  rose  bush  or  other  shrub  be 
girdled  (cutting  away  a  ring  of  bark  down  to  the  cambium 
§  69)  and  the  injured  region  covered  with  wax  to  prevent 
drying,  a  thickened  band  of  bark  will  slowly  develop  above 
the  girdle,  but  never  below.  Evidently  the  tissue  above  the 
girdle  obtains  more  food  for  growth,  and  this  must  come  down 
the  stem  in  the  sieve-tubes  of  the  bark,  for  the  wood  continues 
to  have  the  upward  current  of  water  from  the  soil.  A  plant 
so  treated  will  die  after  some  months,  for  the  parts  of  the 
stem  below  the  girdle  and  also  the  roots  cannot  get  the 
necessary  foods  from  the  leaves. 

Another  example :  Trees  often  die  because  an  iron  wire 
has  been  left  wrapped  around  them  until  the  stem  has  grown 
so  large  that  the  ring  of  wire  becomes  embedded  in  the  bark 
down  to  the  cambium,  thus  cutting  off  the  sieve-tubes  of  the 
bark  and  preventing  foods  from  reaching  the  roots.  Farmers 
often  girdle  trees  in  early  summer  in  order  to  prevent  them 
from  shading  crops;  on  some  kinds  of  trees  the  leaves  will 
remain  green  all  summer  and  sometimes  die  next  year  if 
only  the  bark  is  girdled.  But  even  the  hardiest  trees,  like 
the  honey-locust,  will  fail  to  put  forth  leaves  after  two  or 
three  summers,  because  the  supply  of  food,  which  came  down 
from  the  leaves  and  was  stored  in  the  roots  before  the  girdling, 
is  gradually  used  up,  and  no  more  can  get  down  to  the  roots. 
In  order  to  make  the  leaves  of  such  trees  wilt  soon  after 
girdling,  it  is  necessary  to  chop  through  the  bark  and  several 
inches  into  the  wood.  Anticipating  the  lessons  on  stems  in 
Chapter  VIII,  it  may  be  stated  here  that  only  the  wood-tubes 
in  the  light-colored  outer  layers  of  wood,  known  to  lumber- 
men as  sap-wood,  are  important  in  the  ascent  of  water ;  and 
therefore  cutting  deep  into  the  sap-wood  of  a  tree  will  cut 
so  many  wood-tubes  that  the  leaves  cannot  get  water  to 
supply  loss  by  evaporation,  and  hence  soon  wilt. 

Still  another  interesting  proof  that  the  prepared  foods 
from  the  leaves  go  down  the  stem  in  the  bark  next  to  the 


108  APPLIED  BIOLOGY 

cambium  is  that  a  branch  of  a  grape-vine  girdled  between  a 
bunch  of  grapes  and  the  root  will  produce  better  fruit  on  that 
branch,  because  the  foods  from  the  leaves  are  kept  from  going 
on  down  the  stem.  Of  course,  such  a  girdled  branch  will  be 
useless  in  the  next  fruiting  season,  but  since  grapes  are 
always  developed  on  new  shoots,  it  is  the  common  practice 
to  prune  away  each  winter  most  of  the  branches  which  bore 
fruit  in  the  preceding  summer. 

It  is  possible  for  foods  to  go  up  a  stem.  The  leafy  branch 
above  a  bunch  of  grapes  might  be  cut  off  and  the  grapes 
continue  to  grow  from  foods  coming  up  the  stem  in  the  wood- 
tubes  from  other  branches  with  leaves.  In  the  spring 
before  leaves  appear  on  trees  and  shrubs,  foods  go  up  the 
stem  from  the  cells  in  root  or  stem  where  they  were  stored 
during  the  winter.  If  a  branch  of  a  tree  be  cut  off  in  winter 
just  above  a  bud,  this  bud  will  usually  grow  rapidly  in  the 
following  spring  and  form  a  large  branch.  Orchardists 
make  use  of  this  fact  when  they  prune  trees.  All  these 
cases  prove  that  the  foods  must  be  able  to  go  up  the  stem. 
By  removing  girdles  of  bark  it  can  be  demonstrated  that 
the  upward  movement  is  in  the  wood-tubes. 

It  is  evident  from  the  experiments  described  in  the  above 
paragraphs  that  foods  in  solution  in  water  may  be  transported 
either  up  or  down  the  stem,  depending  upon  where  the  supply 
is  located  and  where  needed. 

104.  Use  of  Foods  Transported  from  Leaves.  —  The 
dissolved  sugars  and  other  more  elaborated  foods  made  in 
the  leaves  may  be  used  in  the  plant  in  two  ways,  as  follows : 

(1)  Used  by  Cells.  —  We  have  seen  that  living  plants  re- 
quire food,  and  this  is  true  of  every  living  cell.  Some  of 
the  food  from  the  leaves  is  at  once  used  by  cells  in  roots, 
stem,  flowers,  or  developing  fruits.  Part  of  the  food  un- 
doubtedly goes  to  make  new  particles  of  living  matter 
(protoplasm)  to  replace  that  continually  being  worn  out  and 
made  lifeless.  It  should  be  kept  in  mind  that  a  living  plant 


AN  INTRODUCTION   TO  PLANT  BIOLOGY         109 

is  like  a  moving  machine  in  that  activity  causes  wear  or 
waste,  and  hence  some  food  must  be  used  for  repair  or  the 
entire  plant  will  soon  die.  In  short,  some  food  must  be 
used  continually  in  making  new  protoplasm.  If  more 
protoplasm  is  made  than  is  needed  for  repair  of  waste,  the 
result  will  be  growth ;  and  usually  growth  means  the  forma- 
tion of  new  cells.  This  is  especially  true  at  the  growing 
tips  of  plant  stems.  The  making  of  new  protoplasm  is  known 
as  assimilation,  or  constructive  metabolism. 

(2)  Stored  in  Cells.  —  If  the  foods  received  from  the  leaves 
are  not  needed  for  immediate  repair,  they  are  stored  in  various 
parts  of  the  plant  (stem,  roots,  leaves,  or  seeds).  In  later 
lessons  (Chapter  VIII)  we  shall  study  various  modifications  of 
plants  adapted  to  storing  foods ;  but  for  our  present  purposes 
it  will  be  sufficient  to  mention  that  carrots,  turnips,  and 
sugar-beets  are  examples  of  plants  that  store  large  quantities 
of  plant  food  in  their  roots ;  sugar-cane  and  sago-palm  store 
food  in  their  stems;  the  head  of  a  cabbage  is  a  bundle  of 
leaves  stored  with  food ;  and  beans,  corn,  and  nuts  are  seeds 
stored  with  food.  These  are  examples  of  various  plant  or- 
gans in  which  food  is  stored  for  the  future  use  of  the  plants. 
It  has  happened  that  man  and  the  herbivorous  (plant-eating) 
animals  have  found  it  convenient  to  appropriate  many  of 
these  reserves  of  plant  food  for  use  in  their  own  food-supply. 

These  reserved  foods  of  the  plant  are  commonly  stored  as 
oil  and  starch,  which  are  easily  seen  (with  low  power  of  micro- 
scope) in  thin  sections  of  various  stems  and  roots,  especially 
in  the  late  autumn  or  winter  after  storage  has  been  going 
on  during  the  entire  growing  season.  In  cases  of  such 
starch  storage,  the  sugar  solutions  derived  from  starch  in 
the  leaves  osmose  into  the  cells  of  the  root,  stem,  or  fruit,  and 
is  then  changed  and  stored  as  starch-grains.  When  the  plant 
needs  this  stored  starch,  the  starch-grains  are  changed  back 
again  into  sugar,  which  is  able  to  osmose  out  of  the  cell  and 
into  other  cells. 


110  APPLIED  BIOLOGY 

But  all  cells  do  not  store  starch.  Sugar,  from  which  our 
ordinary  granulated  sugar  is  obtained,  is  stored  in  large  quan- 
tities in  the  cells  of  sugar-beets  and  in  sugar-cane.  Some- 
times the  sugar  which  enters  cells  is  changed  to  oil,  as  in 
nuts ;  and  when  the  plant  needs  the  oil  elsewhere  it  is  changed 
(sometimes  to  sugar  again)  and  osmoses  out  of  the  cell. 
In  still  other  cases  the  sugar  from  the  leaves  enters  cells  and 
is  used  in  combination  with  nitrogen  and  other  elements  from 
the  soil  to  form  protein  substances. 

Enzymes.  —  All  these  remarkable  changes  which  take 
place  when  foods  are  stored  in  plant  cells  have  long  puzzled 
chemists.  No  one  yet  understands  fully  how  the  plant  cells 
are  able  to  make  these  changes;  but  it  is  known  beyond 
doubt  that  these  changes  do  occur  regularly  in  the  life  of 
plants,  and  further  it  is  known  that  many  peculiar  substances 
called  enzymes  (e.g.,  diastase)  are  present  in  plant  cells,  and 
that  in  some  way  not  understood  and  not  yet  imitated  in 
the  chemist's  laboratory  these  enzymes  can  change  sugar  into 
starch,  oil,  or  albuminous  foods,  or  these  back  into  sugar. 
A  peculiarity  of  enzymes  is  that  they  can  change  other  sub- 
stances without  undergoing  change  themselves,  and  that  a 
small  quantity  of  enzyme  can  produce  a  large  amount  of 
change.  The  most  familiar  example  of  an  enzyme  is  pepsin, 
which  in  the  stomach  of  animals  digests  protein  foods. 

Whatever  the  forms  in  which  foods  may  be  stored,  it 
appears  that  they  are  commonly  in  the  form  of  sugar  when 
passing  (by  osmosis)  into  or  out  of  any  living  cell. 

105.  The  Oxygen-Supply  of  Plants :  Respiration.  —  It 
has  been  pointed  out  that  plants  breathe  or  respire  and  have 
the  same  effect  upon  air  which  animals  have;  namely, 
they  absorb  oxygen  from  the  air  and  give  off  carbon  dioxide 
(062).  The  term  respiration  is  in  both  plants  and  animals 
usually  applied  to  this  combined  process ;  but  it  is  simpler 
to  consider  first  the  obtaining  of  oxygen. 

In  a  plant  without  chlorophyll  (e.g.,  a  mushroom),  oxygen 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         111 

is  absorbed  at  all  times  of  the  day  and  night  by  the  surface 
of  the  plant  in  contact  with  the  air;  and  the  gas  then  diffuses 
or  osmoses  from  cell  to  cell,  for  all  living  cells  must  have  a 
continual  supply  of  oxygen.  If  some  mushrooms  be  placed 
in  a  closed  jar  for  many  hours,  it  can  be  demonstrated  by 
chemical  tests  that  the  air  in  the  jar  has  lost  oxygen.  This 
will  be  the  same  no  matter  whether  the  jar  is  kept  in  light 
or  in  total  darkness. 

A  bean  plant  kept  in  darkness  in  a  closed  jar  will  act  like 
the  mushroom  at  all  times ;  that  is,  it  will  take  up  oxygen. 
But  the  same  plant  kept  in  a  closed  jar  for  several  hours  in 
sunlight  will  give  off  oxygen.  Apparently  this  is  the  reverse 
of  the  mushroom  or  the  green  plant  in  darkness ;  but  let  us 
withhold  our  conclusion  until  we  have  reviewed  previous 
lessons  dealing  with  carbon  dioxide  in  leaves. 

We  have  noted  (§  99)  that  carbon  dioxide  is  used  rapidly  in 
making  carbohydrates  (in  photosynthesis)  while  the  plant 
is  in  light;  but  in  darkness  the  plant  makes  no  carbohy- 
drates and  consequently  uses  no  carbon  dioxide.  Now,  in  the 
combining  of  the  elements  of  carbon  dioxide  and  of  water  to 
make  sugar  or  starch,  the  carbon  of  the  carbon  dioxide  is 
used,  but  the  oxygen  is  not  needed  and  is  set  free  in  the  cells 
of  the  leaves.*  The  amount  of  oxygen  thus  freed  is  very 
much  more  than  the  plant  needs  for  its  oxygen-supply; 
that  is,  it  is  very  much  more  than  the  same  plant  in  darkness 
would  absorb  from  the  air;  and  the  result  is  that  there  is 
excess  oxygen  to  be  given  off  to  the  air.  The  truth  is  that 
the  living  cells  throughout  the  plant  require  oxygen  the  same 
in  light  as  in  darkness ;  but  while  the  starch-making  goes  on 


*  Readers  who  have  studied  chemistry  will  be  interested  in  the  proportions 
of  CO2,  H2O  and  O  in  starch-making  as  follows:  (6  CO2+5  H2O)*  = 
(C6H10O5+602)Z,  which  means  that  to  every  six  molecules  of  carbon  dioxide 
and  five  of  water  there  will  be  one  molecule  of  starch  (C6H10O5)  and  six 
molecules  of  free  oxygen.  This  formula  simply  gives  the  proportions ;  for 
starch  is  some  unknown  multiple  of  C6HioOs. 


112  APPLIED   BIOLOGY 

so  much  oxygen  is  freed  from  carbon  dioxide  that  the  plant 
cannot  use  it  all. 

Oxygen  Liberated  by  Photosynthesis.  —  (/))  Place  some  water  plants, 
such  as  Elodea,  in  a  glass  funnel  which  is  then  placed  with  tube 
upwards  in  a  glass  battery-jar  filled  with  water.  The  water  must 
be  deep  enough  to  more  than  cover  the  funnel  and  its  tube.  Fill 
a  test-tube  with  water,  and  keeping  its  mouth  below  the  water  in 
the  battery-jar,  invert  it  over  the  end  of  the  funnel  tube.  Set  in  sun- 
light. Bubbles  of  gas  (chiefly  oxygen)  will  rise  from  the  plants  and 
displace  the  water  in  the  test-tube.  The  gas  may  be  tested  for 
oxygen  by  stoppering  the  test-tube  before  lifting  from  the  water,  and 
then  quickly  inserting  a  glowing  taper  when  the  stopper  is  removed. 

Critical  study  has  proved  that  all  kinds  of  plants,  as  well 
as  animals,  require  oxygen  constantly.  They  may  get  the 
necessary  amount  directly  from  the  air,  or  green  plants  may 
get  it  from  carbon  dioxide  when  the  carbon  of  that  com- 
pound is  used  in  starch-making  during  daylight.  There  is, 
then,  no  real  difference  between  the  breathing  of  mushrooms 
or  animals  and  plants  with  chlorophyll.  The  increase  in 
oxygen  and  decrease  of  C02  in  the  air  around  green  plants 
during  daylight  is  obviously  due  to  the  independent  process 
of  photosynthesis,  not  to  their  breathing. 

There  are  good  reasons  for  believing  that  roots  of  plants 
absorb  oxygen  from  the  air,  which  is  abundant  in  good  soil. 
In  fact,  one  scientific  reason  for  cultivating  or  tilling  the  soil 
is  to  mix  air  with  the  soil  particles.  The  water  of  the  soil 
contains  oxygen  in  solution,  just  as  the  water  in  a  river 
contains  oxygen  which  fishes  can  absorb  by  their  gills.  When 
the  water  is  taken  up  into  the  plant  stem  it  probably  carries 
along  with  it  some  oxygen,  which  is  absorbed  by  the  cells 
with  which  the  water  comes  into  contact. 

Also,  some  of  the  large  tubes  of  the  fibro-vascular  bundles 
are  filled  with  air,  probably  taken  in  chiefly  at  the  leaf- 
pores  and  also  at  the  bark-pores  (lentides),  which  are  slit- 
like  openings  in  the  bark  leading  to  internal  air-spaces. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY 


113 


The  following  experiment  shows  how  air  may  enter  the  leaf 
and  pass  through  the  stem. 

(D)  Select  a  wide-mouthed  bottle  and  a  cork  or  rubber  stopper 
with  two  holes.  Take  a  leaf  with  a  small  round  petiole  and  push 
the  petiole  into  a  hole  in  the  cork  and  almost 
to  the  bottom  of  a  bottle.  Also  fit  a  glass 
tube  into  the  other  hole  of  the  cork.  Fill 
the  bottle  half  full  of  water.  Use  vaseline 
to  make  the  apparatus  air-tight.  Apply  suc- 
tion to  the  glass  tube  (a  small  bicycle  pump 
with  the  leather  on  the  plunger  reversed,  so 
as  to  "draw"  air  out,  and  connected  by 
rubber  tubing  with  a  reversed  valve  from  a 
bicycle  tire  will  answer,  if  an  air-pump  is 
not  at  hand).  Air  will  exude  from  the  cut 
end  of  the  petiole  and  appear  as  bubbles  in 
the  water.  The  experiment  may  be  reversed 
by  attaching  the  petiole  to  rubber  tubing 
leading  to  a  bicycle  pump,  and  forcing  air 
from  the  pump  into  the  tubes  of  the  petiole  FlGV  39  Apparatus  to 

show  that  air  can  enter 


the  leaf. 
burger.) 


(From  Stras- 


and  out  through  the  leaf,  which  should  be 
held  under  water  so  as  to  make  air-bubbles 
rise  from  the  leaf  surface. 

106.    Excretion   of   Carbon    Dioxide   from  Plants.  —  The 

oxygen  absorbed  by  plants  and  distributed  to  all  their  living 
cells  is  used  in  the  cells  in  a  process  of  slow  oxidation  or 
chemical  union  of  oxygen  with  foods  and  other  substances 
in  the  cells.  This  is  the  same  as  in  animal  cells.  Such 
oxidation  is  constantly  going  on  among  the  particles  of 
living  plant  cells,  and  one  of  the  substances  formed  is  carbon 
dioxide  (CO2).  When  we  remember  that  substances  con- 
taining carbon  are  abundant  in  cells,  we  can  understand  why 
oxidation  of  cell-substances  should  form  a  compound  of  carbon 
and  oxygen  (C02) .  For  example,  when  sugar  is  highly  heated 
the  effect  is  first  to  drive  off  the  water  and  leave  carbon, 
which  then  burns  and  disappears  in  the  air  as  a  gas  (CO2). 
Something  similar  occurs  in  living  cells  when  any  substance 
made  from  carbohydrate  foods  burns.  The  result  is  carbon 


114  APPLIED  BIOLOGY 

dioxide  (CO2)  and  water  (H2O) .  The  water  thus  formed  can- 
not be  distinguished  from  the  other  water  which  is  abundant 
in  plant  cells.  The  carbon  dioxide  is  transported  (probably 
chiefly  in  solution  in  the  moving  liquids  in  plants),  to  the 
surface,  especially  of  the  leaves,  and  then  diffuses  to  the  sur- 
rounding air. 

As  we  have  seen  in  §  105,  a  plant  with  chlorophyll  does 
not  appear  to  give  off  carbon  dioxide  in  light,  because  the 
starch-making  machinery  is  using  that  gas  much  more  rapidly 
than  the  cells  of  the  same  plant  are  making  it.  If  any  carbon 
dioxide  made  in  the  cells  of  the  roots  or  of  the  stem  is  carried 
in  the  water  current  to  the  leaves,  it  may  be  used  in  starch- 
making;  and  in  addition,  the  leaves  must  continually 
absorb  more  of  the  gas  from  the  air.  As  has  been  shown,  a 
closed  jar  containing  a  measured  quantity  of  carbon  dioxide 
and  a  green  plant,  and  placed  in  sunlight,  will  have  less  of 
the  gas  after  a  few  hours;  while  a  similar  jar  and  plant 
kept  in  total  darkness  will  have  more  carbon  dioxide  in 
the  air  of  the  jar.  In  the  first  jar  the  plant  must  have  used 
for  starch-making  all  the  carbon  dioxide  produced  by  oxida- 
tion from  its  own  cells  and  in  addition  some  of  the  gas 
taken  from  the  air. 

107.  Other  Excretions  of  Plants.  —  The  term  excretions 
is  commonly  applied  to  such  substances  as  carbon  dioxide, 
which  are  produced  by  oxidation  in  the  cells  of  plants  and 
animals,  and  which  are  eliminated  because  they  are  of  no 
further  use  in  the  cells,  and  are  sometimes  actually  poisonous. 
Carbon  dioxide  and  water  have  been  mentioned  above  as  two 
excretions  formed  by  oxidation  of  cell-substances  containing 
the  elements  carbon  and  hydrogen.  The  water  formed  by 
oxidation  mixes  with  the  water  taken  into  the  plant  by  the 
roots,  which  is  being  eliminated  continually  by  evaporation. 

In  addition  to  carbon  dioxide  and  water,  plant  cells  form 
other  excretions  and  also  have  an  excess  of  certain  substances 
containing  the  elements  absorbed  with  water  from  the  soil.  In 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         115 

higher  animals  all  excretory  and  excess  substances  containing 
nitrogen,  sulphur,  phosphorus,  calcium,  etc.,  are  in  solution 
in  the  water  eliminated  by  the  kidneys.  Land  plants  usually 
give  off  water  only  by  evaporation,  and  certain  excretory 
and  excess  matters  are  left  behind  in  the  plant  (in  the  leaves) 
just  as  lime  is  left  in  a  tea-kettle.  Plants  living  in  water 
may  have  some  of  these  substances  absorbed  by  the  surround- 
ing water.  Also,  plants  may  eliminate  some  substances  by 
the  roots,  for  the  roots  of  some  plants  (e.g.,  oat  seedlings)  will 
etch  or  corrode  polished  marble  or  limestone  by  the  action  of 
acid  substances  which  osmose  from  the  roots.  Possibly  these 
corroding  substances  enable  a  plant  to  dissolve  useful  minerals. 
Also,  it  has  been  shown  quite  recently  that  grasses  give  off 
from  their  roots  to  the  soil  some  substances  which  poison 
and  check  the  growth  of  young  orchard  trees.* 

It  seems  certain  that  some  of  the  substances  formed  by 
the  oxidation  of  cell-substances  are  stored  in  the  plant, 
but  often  changed  so  as  to  render  them  harmless,  or  even 
useful,  to  the  plant  which  produces  them.  Our  knowledge  of 
a  large  number  of  peculiar  substances  found  in  plants  is  still 
very  incomplete;  but  it  seems  probable  that  many  of  them 
are  modified  and  stored  waste  products  which  could  not  be 
eliminated  along  with  water  as  animals  eliminate  similar 
substances  by  the  kidneys.  Examples  of  such  substances 
which  are  not  known  to  be  of  further  use  to  the  plants 
which  contain  them,  are  the  active  constituents  of  tea  (in 
leaves),  coffee  (in  seed),  cocoa  (in  seeds),  the  poisonous  sub- 
stance in  the  pits  (seeds)  of  almonds,  peaches,  and  plums; 
quinine  from  the  bark  of  a  tree;  nicotine  in  tobacco  plants; 
strychnine  and  many  other  poisons  in  various  plants.  These 
and  many  other  peculiar  products  of  plants  contain  the  ele- 
ment nitrogen,  and  some  of  them  are  chemically  similar  to  the 


*  For  an  account  of  this  see  bulletins  published  by  U.  S.  Dept.  of  Agri- 
culture. 


116  APPLIED  BIOLOGY 

nitrogenous  excretions  (those  containing  nitrogen)  formed  in 
animals.  While  the  plant  cannot  eliminate  these  substances 
directly,  it  is  interesting  to  note  that  many  of  them,  along 
with  mineral  substances  taken  up  with  water  from  the  soil, 
become  stored  in  leaves,  fruit,  seeds,  and  bark  —  all  of 
which  parts  of  plants  are  frequently  detached,  resulting 
in  the  elimination  from  the  plants  of  useless  and  possibly 
sometimes  harmful  substances.  In  many  other  cases  sub- 
stances of  no  further  use  to  the  plant  may  be  stored  per- 
manently in  harmless  forms  in  stem  or  roots. 

This  habit  of  storing  certain  useless  substances  is  peculiar 
to  plants.  As  plants  grow  older  the  amount  of  stored  sub- 
stance, especially  mineral  matter  from  the  soil,  increases. 
Thus  a  leaf  collected  and  burned  in  early  summer  leaves 
little  ash  as  compared  with  a  similar  one  taken  in  late  autumn. 
Throughout  the  growing  season  water  has  been  evaporating 
and  leaving  behind  in  the  leaves  the  mineral  materials  carried 
up  from  the  soil. 

Animals  do  not  store  excess  or  useless  mineral  substances 
and  excretions,  but  eliminate  them  daily,  dissolved  in 
water  which  is  discharged  by  means  of  the  kidney-system. 
Since  most  plants  cannot  discharge  water  in  liquid  form, 
storage  of  useless  mineral  matter  and  some  excretions  is 
the  plant's  only  possible  way  of  doing  the  same  kind  of 
necessary  work  which  the  kidneys  of  animals  do. 

108.  Irritability  of  Plants.  —  Irritability  in  either  plants  or 
animals  is  the  power  of  responding  to  a  stimulus.  For  ex- 
ample, if  a  frog  be  touched  suddenly  (mechanical  stimulus), 
the  leg  muscles  contract  and  the  animal  jumps.  The  frog  also 
responds  to  heat  stimulus  (goes  into  shade  when  the  sun's 
heat  is  too  great) ;  to  light  stimulus  (sees  enemies  and  jumps) ; 
to  sound  stimulus  (hears  sounds  and  jumps) ;  electrical 
stimulus  (jumps  if  touched  by  a  slightly  charged  electric 
wire).  These  are  phases  or  kinds  of  irritability  common  in 
animals  which  have  a  brain,  spinal  cord,  and  nerves. 


AN  INTBODUCTION  TO  PLANT  BIOLOGY         117 

We  shall  later  study  some  microscopic  animals  which 
respond  to  mechanical,  heat,  light,  and  electrical  stimuli ; 
but  they  have  no  visible  nerves  or  nerve  organs.  These 
lower  animals  have  irritability  or  nervous  functions  without 
special  organs  to  perform  the  functions.  This  is  essentially 
the  case  in  plants.  In  recent  years  there  have  been  many 
magazine  articles  discussing  "  the  nerves  of  plants."  The 
truth  is  that  no  one  has  seen  any  nerves  or  brains  or  similar 
nervous  organs  in  plants,  although  they  do  exhibit  the 
various  forms  of  irritability  and  respond  to  the  different 
kinds  of  stimuli  which  affect  animals,  as  the  following  ex- 
amples will  show :  — 

Mechanical  or  touch  stimuli  (such  as  a  jar  by  passing 
animals)  causes  the  sudden  folding  of  the  leaflets  and  drooping 
of  the  branches  of  the  Mimosa  or  sensitive  plant  (see  Fig.  1). 
The  leaf  of  the  Venus  fly-trap  (Fig.  2)  closes  quickly  and 
grasps  insects  which  happen  to  touch  it. 

Response  to  light  stimulation  is  seen  in  the  familiar  grow- 
ing of  house-plants  toward  the  window,  and  also  in  the 
opening  and  closing  of  many  flowers.  Flowers  of  the  dande- 
lion and  others  of  the  same  family,  California  poppy,  etc., 
open  in  the  light  and  close  when  placed  in  darkness. 

Heat  stimulus  also  causes  many  flowers  to  open  and  close ; 
tulip,  crocus,  and  "  star  of  Bethlehem  "  are  familiar  examples. 
The  combined  effect  of  heat  and  light  stimuli  upon  the  open- 
ing of  flowers  is  so  marked  in  many  species  of  plants  that  it 
is  possible  to  make  a  flower-clock,  which  is  simply  a  garden- 
bed  arranged  to  imitate  a  clock-dial,  in  which  for  each  hour 
of  daylight  there  is  a  selected  group  of  plants  whose  flowers 
are  commonly  open  at  that  hour.  Of  course,  the  flowers  do 
not  open  exactly  on  the  hour,  for  the  controlling  temperature 
and  sunlight  vary  from  day  to  day.  However,  many  flowers 
are  open  in  early  morning ;  certain  flowers  (example,  "  ten- 
o'clock  ")  open  in  the  middle  of  the  forenoon ;  others  (like 
"  flower-of-an-hour  ")  open  only  in  the  mid-day  sunshine ; 


118  APPLIED  BIOLOGY 

"  four-o'clock  "  and  others  open  in  late  afternoon;   and  the 

primrose  at  sunset. 


P.< 
1 


Home-work:  Keep  a  list  of  the  plants  which  you  have  an  op- 
portunity to  observe,  and  note  the  hours  when  the  flowers  are  seen 
open.  ' 

Leaves  of  many  plants  (oxalis,  clovers,  bean,  etc.)  droop 
or  fold  in  darkness  and  assume  the  so-called  " sleep"  position. 
Some  so-called  "  compass  plants  "  avoid  the  intense  noonday 
sun  by  moving  their  leaves  so  that  the  edges  are  vertical  and 
in  the  north-south  direction. 

Another  form  of  external  stimulus  affecting  plants  is  that 
of  gravitation.  That  the  stems  of  most  plants  ordinarily 
grow  upward  and  the  roots  downward  is  a  familiar  fact. 
Experiments  made  by  growing  young  plants  attached  to  ro- 
tating wheels  prove  that  this  direction  of  growth  of  plants 
in  a  state  of  nature  is  due  to  gravitation. 

Plants  also  respond  to  water.  Roots  will  turn  away 
from  dry  soil  and  grow  in  the  direction  of  greater  moisture. 

Still  other  forms  of  the  responses  of  plants  are  the  numer- 
ous cases  of  the  twining  of  stems  and  the  movements  of 
tendrils  and  special  roots  in  order  to  aid  in  climbing. 

Tropisms.  —  We  see  that  in  many  different  ways  plants 
have  irritability  and  respond  to  external  stimuli.  The  re- 
sponses in  plants  are  much  slower  than  in  animals,  but  they 
are  none  the  less  definite.  These  reactions  of  plants  to 
stimuli  are  often  known  as  tropisms  (from  a  Greek  word 
meaning  to  turn).  Turning  in  response  to  light  is  helio- 
tropism  (literally,  turning  to  the  sun),  or  phototropism  ; 
to  heat  is  thermotropism ;  to  gravity  is  geotropism  (literally, 
turning  to  the  earth) ;  to  water  is  hydrotropism ;  to  chemicals 
is  chemotropism ;  to  electricity,  which  seems  to  have  little 
influence  on  plants  in  nature,  is  electrotropism.  The  same 
terms  are  used  in  describing  the  reactions  of  animals  to  the 
various  kinds  of  stimuli. 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         119 

Although  these  responses  of  plants  to  various  stirmili 
have  been  observed  and  studied  for  a  long  time,  we  d 
understand  them.  Neither  do  we  know  why  a  frog  respo1 
when  touched.  We  simply  know  that  it  has  the  power  of 
such  response  and  that  in  some  way  it  is  connected  with  the 
protoplasm  in  certain  special  nervous  organs  of  the  frog. 
Likewise,  we  know  many  facts  about  plants  responding  in 
ways  very  similar  to  animals  and  that  this  power  of  response 
is  connected  with  the  protoplasm  of  plants,  but  apparently 
in  no  particular  organs  like  nervous  organs.  We  use  the  word 
irritability  to  mean  that  animals  and  plants  respond  to 
stimuli.  We  do  not  know  what  it  is,  but  there  is  plenty  of 
evidence  that  such  a  power  exists,  and  we  know  much  about 
how  it  works.  Likewise,  we  know  that  electricity  exists, 
very  much  about  how  it  acts,  and  can  make  great  use  of 
it ;  and  yet  no  one  knows  what  electricity  is.  These  are 
merely  examples  out  of  hundreds  of  cases  in  science  where 
we  must  accept  the  facts  and  make  use  of  the  knowledge 
which  it  is  possible  to  discover  even  when  we  cannot  find 
a  satisfactory  explanation. 

109.  Summary  of  Work  of  Plant  Organs  :  Life-Processes. 
—  Most  plants  require,  as  food-material,  carbohydrates, 
nitrogen  compounds,  and  certain  other  elements.  Plants  with 
chlorophyll  can  make  the  necessary  carbohydrates  by  com- 
bining the  elements  from  carbon  dioxide  and  water.  Light 
and  chlorophyll  are  essential  for  making  carbohydrates. 
Plants  which  have  no  chlorophyll  must  absorb  the  carbo- 
hydrates (probably  as  sugar)  from  decaying  organic  matter. 
The  nitrogen,  in  the  form  of  compounds  with  other  necessary 
elements,  are  commonly  absorbed  from  the  soil  or  water  in 
which  the  roots  grow.  ' 

All  plants  require  oxygen  for  use  in  the  oxidation  which  is 
going  on  constantly  in  all  living  cells.  Plants  with  chlorophyll 
may  get  oxygen  from  that  which  is  freed  from  carbon  dioxide 
when  the  carbon  is  used  in  starch-making.  For  this  reason 


120  APPLIED  BIOLOGY 

such  plants  do  not  appear  to  take  oxygen  in  the  daytime  from 
the  surrounding  air,  as  they  do  at  night,  and  as  plants  with- 
out chlorophyll  do  both  day  and  night. 

All  plants  produce  excretions  by  the  oxidation  going  on  in 
their  cells.  Most  prominent  of  these  is  carbon  dioxide, 
which  all  plants  give  off  at  night  to  the  air  or  water  in  which 
they  live.  Plants  without  chlorophyll  give  off  carbon  dioxide 
in  daylight  also ;  but  the  green  plants  use  this  gas  so  rapidly 
when  making  starch  that  during  the  daytime  none  appears 
to  be  given  off  to  the  surrounding  air. 

All  plants  have  assimilation  or  constructive  metabolism 
of  some  foods  into  new  protoplasm.  This  takes  place  only 
in  living  cells.  Much  of  the  food  containing  only  carbon, 
hydrogen,  and  oxygen  (i.e.,  carbohydrates  and  oils)  is  be- 
lieved to  undergo  oxidation  in  cells  or  is  stored  for  future  use, 
but  does  not  become  protoplasm.  Probably  only  a  small 
part  of  the  contents  of  an  ordinary  plant  cell  is  living  matter, 
and  much  of  the  cell-substance  which  we  see  with  the  micro- 
scope consists  of  food-materials,  water,  and  other  lifeless 
substances. 

Digestion  of  foods  is  necessary  whenever  insoluble  foods 
(starch,  oil,  proteins)  require  transfer  from  cell  to  cell  or  to 
distant  organs  of  the  plant. 

Moving  liquids  in  the  higher  plants  serve  to  transport  the 
foods,  oxygen,  and  excretions ;  but  these  liquids  do  not  make 
a  complete  circuit  as  do  the  animal  circulating  liquids  (blood 
and  lymph),  whose  function  is  also  transportation  of  foods, 
oxygen,  and  excretions  (§  52). 

Some  form  of  irritability,  or  power  of  responding  to  stimuli, 
is  present  in  all  plants.  But  there  are  no  special  nervous 
organs,  such  as  are  connected  with  irritability  in  higher 
animals. 

All  plants  have  the  power  of  reproduction,  either  from  parts 
of  their  bodies  which  can  grow  into  complete  plants  (asexual 
reproduction),  or  from  egg-cells  which  usually  require  union 


AN  INTRODUCTION  TO  PLANT  BIOLOGY         121 

with  other  cells  (i.e.,  fertilization  by  cells  derived  from  pollen- 
grains)  before  developing  into  new  plants. 

110.  Classification  of  the  Bean  Plant.  —  By  this  we  mean 
the  relation  of  the  bean  plant  to  other  plants.  In  the  first 
place,  the  bean  plant  is  a  member  of  the  great  division  of 
flowering  plants  or  seed-plants.  Within  this  division  are 
many  families,  one  of  which  is  the  family  of  leguminous  plants 
(including  peas,  beans,  vetches,  clovers,  locusts,  and  numerous 
other  plants  which  have  an  irregular  flower  similar  to  that 
of  the  bean  plant).  In  this  family  are  included  a  number 
of  bean-like  plants  which  are  somewhat  different  from  ordi- 
nary garden  beans  (e.g.,  Windsor  beans  and  horse  beans). 
The  common  garden  beans  belong  to  the  genus  Phaseolus 
and  to  the  species  vulgaris,  hence  the  scientific  name  is  Phaseo- 
lus vulgaris.  The  various  kinds  of  ordinary  beans  are  varieties 
of  the  same  species.  The  Windsor  and  horse  beans  belong 
to  another  genus. 

The  meaning  of  classification  of  animals  and  plants  will 
be  discussed  in  Chapter  VII. 

\ 


CHAPTER  VI 


COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY 

IN  an  earlier  lesson  (§  29)  we  noticed  that  living  animals 
and  plants  have  certain  characteristics  which  lifeless  things 
do  not  have.  Most  evident  of  these  characteristics  are  move- 
ment, taking  food  and  assimilating  it,  breathing,  reproducing. 
We  have  now  studied  these  and  other  activities  more  carefully, 
and  are  ready  to  compare  the  animal  and  the  plant.  With- 
out careful  study,  a  frog  and  a  bean  plant  seem  to  have 
nothing  in  common,  except  that  they  are  both  living;  but 
a  detailed  study  has 

shown  us  the  follow- 
ing remarkable  simi- 
larity between  the 
animal  and  the  plant. 
111.  Similarity  of 
Structure  of  Bodies  of 
Animals  and  Plants. — 
In  both  the  frog  and 
the  bean  the  essential 
living  substance  is 
protoplasm,  and  this 
is  found  in  units  of 
structure  called  cells 
(see  Figs.  40,  41). 
Chemical  analysis  has 
shown  no  essential  difference  between  animal  and  plant  pro- 
toplasm. Both  animal  and  plant  cells  have  nuclei,  and  they 
multiply  by  an  automatic  process  of  division.  Biologists 

122 


FIG.  40.  Epidermis  of  salamander  tadpole. 
Three  cells  undergoing  division.  (From 
Wilson.) 


COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY     123 


FIG.  41.  Epidermis  from  leaf  of  onion  bulb. 
Seven  cells  are  in  various  stages  of  division. 
(From  Wilson,) 


whose  specialty  is  the  study  of  cells  are  continually  mak- 
ing discoveries  with  the  microscope  which  make  still  more 
impressive  the  similarity  of  cells  in  animals  and  plants. 

In  both  animals  and 
plants  the  cells  are 
grouped  to  form  tissues, 
and  the  tissues  to  form 
organs,  which  are  simply 
groups  of  cells  for  per- 
forming a  particular 
work  necessary  in  the 
life  of  the  living  thing. 

112.  Food  of  Animals 
and  Plants.  —  Both  re- 
quire food  containing 
the  elements  found  in 
their  bodies.  That  the  necessary  elements  are  the  same  is 
evident  from  chemical  analysis  and  from  the  fact  that  many 
animals  (the  herbivorous)  live  on  plants  as  food.  Both  ani- 
mals and  plants  require  foods*  containing  carbon,  hydrogen, 
and  oxygen.  Plants  which  have  no  chlorophyll,  and  all  ani- 
mals, must  depend  for  such  food  upon  the  sugar,  starch,  and 
oils  which  have  been  made  from  carbon  dioxide  and  water  by 
green  plants  (§  99) .  Herbivorous  animals  get  carbohydrates 
and  oils  directly  by  eating  plants ;  but  carnivorous  animals 
get  similar  food  from  the  fat,  sugar,  and  other  substances  in 
the  flesh  of  herbivorous  animals.  Thus  when  a  frog  eats  an 
earthworm  it  gets  materials  which  the  worm  got  from  its 
plant  food.  In  short,  all  animals,  and  plants  like  the  mush- 
rooms, depend  upon  those  plants  which  are  able  to  combine 
the  elements  of  carbon  dioxide  (C02)  and  water  (H20)  to 
form  such  foods  as  starch,  sugar,  and  fat. 

In  addition  to  foods  containing  only  three  elements  (C,H,0), 
animals  and  plants  require  foods  containing  also  the  element 
nitrogen.  Most  plants  are  able  to  get  nitrogen  by  using 


124  APPLIED  BIOLOGY 

very  simple  substances  absorbed  from  water  in  soil  (e.g., 
sodium  nitrate  used  as  a  soil  fertilizer),  and  to  unite  ele- 
ments from  these  with  those  of  sugar  to  make  the  substance 
known  as  protein  or  albumen,  which  contains  nitrogen  in 
addition  to  carbon,  hydrogen,  and  oxygen.  Most  plants 
can  do  this. 

Some  very  simple  plants  (bacteria)  can  use  free  nitrogen 
from  the  air.  Animals,  on  the  other  hand,  must  get  their 
nitrogenous  (nitrogen-containing)  food  in  the  form  of 
proteins,  for  no  animal  has  the  power  to  make  these  from 
carbohydrates  and  such  simple  substances  as  plants  get  from 
the  soil.  Hence  all  animals  depend  upon  plants  for  their 
protein  food,  either  getting  it  directly  by  eating  plants  as 
food  or  indirectly  by  eating  flesh  from  animals  which  derive 
their  protein  from  plants. 

Animals,  then,  depend  upon  plants  for  all  their  food-supply. 
They  require  as  foods  carbohydrates,  fats,  and  protein;  and 
only  plants  are  known  to  make  these. 

Especially  should  we  note  that  both  animals  and  plants 
depend  upon  sunlight,  which  is  necessary  for  combining 
elements  from  carbon  dioxide  and  water  into  carbohydrates, 
which  in  turn  are  necessary  for  making  proteins.  Hence  the 
energy  manifested  in  the  activities  of  animals  comes  from 
food  formed  by  plants  and  indirectly  from  sunlight.  This 
is  what  is  meant  by  the  statement  in  popular  books  that  the 
"  energy  in  foods  is  stored  sunlight." 

Stored  Energy.  —  It  should  be  noted  further  that  plants  by 
photosynthesis  of  carbohydrates  store  energy  which  may  later 
be  converted  (by  oxidation)  into  energy  of  action  by  either 
plants  or  animals.  Besides  foods,  we  may  name  as  examples 
of  stored  energy  such  cases  as  gunpowder,  a  waterfall,  coal, 
a  wound-up  watch-spring,  etc.  Such  stored  energy  is  said 
to  be  potential;  that  is,  it  has  latent  power.  Energy  being 
liberated  from  burning  coal,  exploding  powder,  etc.,  is  called 
kinetic  or  energy  of  action. 


COMPARISON   OF  ANIMAL  AND  PLANT  BIOLOGY     125 

All  the  above  applies  to  our  human  food-supply,  for  our 
dependence  upon  plants  as  food-producers  is  exactly  the  same 
as  that  of  all  animals.  We  may  eat  food  from  plants,  or  we 
may  eat  meat  from  animals  which  acquired  their  store  of 
foods  from  plants.  So,  directly  or  indirectly,  all  our  food 
comes  from  plants  and  depends  upon  the  action  of  sunlight 
in  the  photosynthetic  processes  of  green  plants. 

113.  Oxygen  Required  by  both  the  Animal  and  the  Plant. 

—  Oxygen  is  necessary  in  all  living  cells,  because  oxidation 
of   cell-substances,    chiefly   absorbed   foods,    goes   on   con- 
tinuously as  long  as  the  animal  or  the  plant  lives.     In  other 
words,  oxidation  in  cells  is  inseparably  connected  with  life- 
activities.     In  both  animals  and  plants  there  are  special 
breathing  mechanisms  for  getting  the  required  oxygen  from 
the  air  and  for  distributing  it  throughout  their  bodies.     (De- 
scribe  these   mechanisms   in   frog   and   bean   plant.)     The 
nitrogen  of  the  air  seems  to  play  the  part  of  a  diluting  sub- 
stance, for  oxidation  goes  on  too  rapidly  when  an  animal  or 
a  plant  is  placed  in  a  jar  of  pure  oxygen.     Animals  require 
more  oxygen  in  proportion  to  weight  than  do  plants,  because 
in  animals  life-activities   (especially  movement),  are  more 
intense,  and  oxidation  is  more  rapid. 

114.  Metabolism  Occurs  in  Both  Animal  and  Plant  Cells. 

—  In  each  case  foods  which  reach  the  cells  may  be  oxidized 
to  furnish  energy,  or  may  be  used  in  forming  new  particles 
of  protoplasm  (repair  and  growth),  or  may  be  stored  until 
needed.     Plant  cells  require  less  food  than  do  animal  cells 
for  supplying  energy  and  for  repairing  protoplasm;    and 
hence  more  is  available  for  growth  and  for  storage.     This 
enables  plants  to  grow  rapidly  and  to  store  up  great  quantities 
of  foods  (sugar,  starch,  proteins,  oils).     Animals  take  ad- 
vantage of  this  food  stored  by  plants.     The  great  difference 
between  animal  and  plant  metabolism  is  that  plant  proto- 
plasm can  make  such  foods  as  sugar,  starch,  oils  and  proteins 
from  simple  materials  like  carbon  dioxide,  water,  and  nitrates. 


126  APPLIED  BIOLOGY 

This  is  constructive  metabolism.  On  the  other  hand,  animal 
protoplasm  cannot  construct  foods  out  of  such  simple  ma- 
terials ;  but  can  only  oxidize  the  foods  made  by  plants.  This 
is  destructive  metabolism,  and  its  result  is  the  formation  of 
excretions.  Plants  do  oxidize  a  limited  portion  of  the  foods 
they  construct,  and  thus  they  form  some  excretions  (e.g., 
CO2  and  H20)  similar  to  those  formed  when  animals  oxidize 
foods.  On  the  whole,  however,  plants  have  a  limited  amount 
of  destructive  metabolism,  and  they  are  characteristically 
constructive. 

Heat.  —  It  should  be  noted  that  oxidation  in  plant  cells,  as 
well  as  in  animals,  produces  heat ;  but  the  amount  is  small 
and  not  evident  except  in  careful  experiments. 

115.  Excretions   of  Animals   and  Plants.  —  Since  oxida- 
tion is  less  rapid  in  plants,  they  produce  less  excretions  than 
animals  do.     Since  both  animals  and  plants  must  oxidize 
foods  made  by  plants,  it  is  to  be  expected  that  the  excretions 
of  animals  and  plants  will  contain  the  same  elements.     As 
we  have  seen,  the  most  evident  excretions  of  animals  and 
plants  are  carbon  dioxide,  water,  nitrogenous,  and  mineral 
substances.     These  contain  all  the  elements  which  a  chemist 
could  find  by  analyzing  the  cell-substance  of  either  animals 
or  plants. 

The  Cycle  of  Organic  Matter.  —  Since  plant  cells  are  primarily 
constructive  in  their  metabolism,  they  are  able  to  use  the 
excretions  formed  by  animals,  which  oxidize  foods  derived 
from  plants;  or  plants  can  use  substances  similar  to  excre- 
tions which  are  formed  by  micro-organisms  from  decaying 
animal  and  plant  matter.  Hence  simple  matter  is  constantly 
being  built  into  foods  by  plants  and  reduced  to  simple  con- 
ditions by  animals. 

116.  The  Cycle  of  Carbon.  — Some  of  the  carbon  dioxide 
in  the   air   comes  from   the    breathing    of    animals;    some 
comes  from  decaying   of   dead  plant  and  animal   matter; 
and  some  also  from  the  burning  of  wood,  oil,  and  coal  (all 


COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY     127 

of  which  fuels  are  of  organic  origin).  Thus  there  is  a  never- 
ending  cycle  of  carbon  dioxide  being  taken  up  from  the  air, 
built  up  into  plant  substance,  which  in  turn  is  oxidized  or  be- 
comes food  for  animals,  then  is  oxidized  to  form  excretions 
or  built  into  animal  substance,  which  ultimately  dies  and 
decays  and  is  again  taken  up  by  the  plant.  In  short,  the 
foods  made  in  plants  are  by  animals  changed  back  into  ex- 
cretions which  plants  use  over  again  in  making  new  foods. 

Without  chlorophyll  and  sunlight  much  of  the  carbon 
dioxide  in  the  world  now  stored  in  the  organic  compounds  of 
plants  and  animals  would  ultimately  become  free  in  the  air, 
and  be  so  abundant  as  to  poison  all  life.  Certainly  all  ani- 
mals would  perish  because  of  the  lack  of  the  foods  which  plants 
with  chlorophyll  must  make  primarily  for  themselves,  and 
which  secondarily  are  used  by  other  plants  and  also  by  ail 
animals.  The  cycle  of  carbon,  then,  is  as  follows :  Car- 
bon from  the  carbon  dioxide  in  the  air  enters  into  the 
composition  of  plant  substances.  This  may  be  oxidized  (in 
the  living  plant)  or  decayed  (in  the  dead  plant)  directly 
back  to  carbon  dioxide,  which  gets  free  in  the  air ;  or  the  plant 
substance  may  be  used  by  animals  and  thence  oxidized  (in 
living  animals)  or  decayed  (after  death  of  the  animal's  body) 
back  to  free  carbon  dioxide. 

Since  animals  depend  upon  plants  in  this  food  relation, 
philosophical  biologists  have  concluded  that  plants  with 
chlorophyll  must  have  existed  on  this  earth  before  animals 
did.  In  fact,  with  the  exception  of  a  few  of  the  lowest  bacteria 
(a  group  of  microscopic  plants),  all  animals  and  plants  are 
dependent  upon  the  plants  which  are  able  to  use  light  and 
carbon  dioxide  in  making  foods.  Some  of  the  simplest 
bacteria  can  make  food  materials  without  light  and  from 
purely  inorganic  materials.  Obviously  such  organisms  could 
have  lived  on  this  earth  before  all  other  living  things  which 
we  know  and  at  a  time  when  there  were  conditions  which 
were  not  adapted  to  plants  with  chlorophyll. 


128  APPLIED  BIOLOGY 

117.  The  Cycle  of  Nitrogen.  —  Parallel  with  the  cycle  of 
carbon  from  air  to  plants,  from  plants  to  animals,  and  from 
animals  back  to  the  air,  there  is  a  corresponding  cycle  of  the 
element  nitrogen.  There  is  an  abundance  of  this  in  the  air, 
but  only  certain  species  of  bacteria  can  use  it  in  its  free  form. 
It  has  been  pointed  out  that  ordinary  plants  get  nitrogen 
in  compounds  absorbed  from  the  soil  and  use  this  in  making 
proteins.  Also,  only  plants  can  make  the  proteins  required 
by  animals  as  food.  Proteins  in  animals  and  plants,  then, 
depend  upon  the  nitrogen  in  simple  compounds  such  as  are 
found  in  fertile  soil.  The  nitrogen  in  the  bodies  of  plants 
and  animals  gets  back  to  the  soil  either  by  the  decay  of  dead 
animals  and  plants,  or  as  excretions  produced  by  animals ; 
while  animals  obtain  protein  foods  from  plants  directly  or 
indirectly  through  other  animals.  Animal  excretions  fur- 
nish nitrogen  to  the  soil,  from  the  soil  the  nitrogen  goes  to 
plants,  from  plants  it  goes  to  animals  as  protein  food,  and 
then  animals  reduce  the  food  to  nitrogenous  excretions, 
which  in  the  soil  may  again  be  taken  up  by  plant  roots. 

Such  is  the  cycle  of  nitrogen,  which,  like  that  of  carbon, 
must  be  continuously  undergoing  repetition.  If  dead  ani- 
mals and  plants  did  not  decay  and  living  animals  not  change 
protein  foods  to  nitrogeneous  excretions,  all  the  once  avail- 
able nitrogen  would  soon  be  bound  in  the  bodies  of  dead  ani- 
mals and  plants  and  life  would  cease.  A  marked  tendency 
toward  such  a  result  is  evident  in  certain  agricultural  regions 
where  farmers  have  wasted  manures  (animal  excretions) 
until  the  soil  has  lost  its  primitive  fertility;  that  is,  has  suf- 
fered exhaustion  of  the  nitrogen  compounds. 

"118.  Interdependence  of  Animals  and  Plants.  —  We  have 
noted  that  animals  depend  upon  food  from  plants  for  both 
carbon  and  nitrogen,  and  that  animals  return  these  elements 
to  plants.  Thus  the  cycle  of  organic  matter  is  a  double 
one,  including  a  cycle  of  carbon  and  one  of  nitrogen. 

The  explanation  of  a  "  balanced  "  aquarium,  one  in  which 


COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY     129 

water  snails  and  aquatic  plants  live  together  for  a  long  time 
without  change  of  water  or  addition  of  food,  is  to  be  found 
in  the  existence  of  the  double  cycle  of  organic  matter.  The 
snails  feed  on  the  plants.  (Fishes  which  would  eat  the  plants 
might  be  kept  in  the  same  way.)  The  carbon  dioxide  excreted 
by  the  snails  is  used  by  the  plants  in  making  carbohydrates. 
Plants  also  absorb  from  the  water  the  nitrogenous  excretions 
of  the  snails.  Thus  the  excretions  of  the  snails  furnish  the 
important  elements,  carbon  and  nitrogen,  for  the  growth  of 
the  plants.  When  the  plants  use  the  carbon  dioxide  ex- 
cretion in  starch-making  there  is  an  excess  of  oxygen,  and 
this,  absorbed  by  the  water,  is  breathed  by  the  snails.  A 
small  quantity  of  the  oxygen  in  the  water  is  absorbed  by  the 
plants,  especially  when  in  darkness.  The  plants  and  the 
snails  when  properly  balanced  in  an  aquarium  so  supplement 
each  other  that  they  may  live  together  indefinitely. 

Such  a  "  balanced  "  aquarium  wherein  animals  and  plants 
supplement  each  other  is  a  sort  of  miniature  of  the  world  in 
general,  for  all  plants  and  animals  have  the  relations  of  food, 
oxygen,  and  excretions  illustrated  by  the  plants  and  animals 
in  the  aquarium. 

119.  Irritability  or   Nervous   Reaction.  —  We  have  pre- 
viously noted  that  higher  animals  have  a  system  of  nervous 
organs;  but  many  of  the  lowest  animals  and  all  plants  have 
no  special  organs  corresponding  to  the  nervous  organs  of 
higher  animals  in  which   are   concentrated  the  powers  of 
responding  to  stimuli.     Nevertheless,  there  is  in  all  living 
things  the  power  of  responding  to  various  kinds  of  stimuli. 
This  power  is  known  as  irritability,  and  it  seems  to  be  one 
of  the  fundamental  characteristics  of  all  living  matter. 

120.  Digestion  occurs  in  both  animals  and  plants,  and  in 
each  case  it  is  a  process  of  rendering  foods  soluble  in  water 
in  preparation  for  osmosis  through  cell-walls.     In  both  plants 
and  animals  digestion  is  accomplished  by  peculiar  substances 
(enzymes),  and  there  is  a  great  similarity  between  these  sub- 

K 


130  APPLIED  BIOLOGY 

stances.  Thus  the  enzyme  (diastase),  which  digests  starch 
to  sugar  in  plant  cells,  is  very  similar  to  enzymes  secreted  in 
the  salivary  glands  and  pancreas  of  higher  animals,  and 
which  digest  starchy  food  to  sugar  in  preparation  for  ab- 
sorption. Both  in  plants  and  animals  there  are  enzymes 
which  have  the  power  to  change  starch,  sugar,  oil  or  fat,  and 
proteins  in  various  ways  to  fit  them  for  absorption,  trans- 
portation, or  storage. 

121.  Water  as  a  transporting  medium  for  foods,  oxygen, 
and  excretions  plays  an  important  part  in  the  life  of  higher 
animals  and  plants,  being  the  basis  of  the  blood  in  animals  and 
of  sap  in  plants.  In  each  of  these  liquids,  water  is  the  medium 
in  which  are  dissolved  the  materials  to  be  transported.  The 
blood  of  animals  contains  more  oxygen  and  excretions  than 
the  same  amount  of  sap  of  any  plant,  because  greater  activity 
in  animals  means  that  more  oxygen  is  required,  and  more 
oxidation  means  that  more  excretions  are  produced.  The 
amount  of  dissolved  food  in  a  given  quantity  of  sap  of  a  plant 
and  blood  of  an  animal  might  be  the  same,  that  is,  as  much 
sugar  or  other  substance  as  can  be  dissolved  in  water  at  a 
given  temperature. 

There  is  one  striking  difference  between  the  transporting 
liquids  in  animals  and  in  plants;  namely,  that  in  animals  the 
liquids  (blood  and  lymph)  move  in  a  circuit  from  the  heart 
around  through  the  tissues  of  the  body  and  thence  back  to 
the  heart.  Hence  there  is  a  circulation  of  the  blood  in  most 
of  the  animals  which  have  blood,  and  there  is  an  organ  (heart) 
for  keeping  up  the  circulation.  In  plants,  on  the  contrary, 
there  is  no  circulation.  Water  from  the  soil  moves  up  in  the 
wood-tubes  of  plants,  and  water  holding  in  solution  foods 
and  other  matters  moves  down  in  the  inner  bark  (sieve-tubes) 
of  plants ;  but  there  is  no  direct  connection  between  the  two 
sets  of  tubes,  and  hence  no  possibility  of  a  complete  circuit. 
However,  the  important  point  is  that  both  plants  and  ani- 
mals use  water  for  transporting  foods,  oxygen,  and  excretions ; 


COMPARISON  OF  ANIMAL  AND  PLANT  BIOLOGY     131 

and  whether  the  water  moves  in  a  circuit,  as  in  animals,  or  up 
and  down,  as  in  plants,  is  a  matter  of  secondary  importance. 
The  complete  circuit  in  animals  has  for  them  the  advantage 
that  a  swifter  movement  of  water  (of  the  blood)  makes 
possible  more  rapid  distribution  of  foods,  oxygen,  and  ex- 
cretions. But  plants,  because  of  the  sluggish  life-activities 
of  their  cells,  can  get  along  with  a  less  rapid  distribution  of 
the  necessary  foods  and  oxygen,  and  excretions. 

We  see,  then,  that  while  sap  in  our  common  plants  and 
blood  in  animals  appear  at  first  sight  to  be  widely  different, 
careful  study  of  their  respective  functions  shows  that  they 
are  really  doing  similar  work  in  connection  with  the  life- 
activities  of  protoplasm. 

122.  Reproduction.  —  This  process  has  many  similarities 
in  animals  and  plants.  The  frog  and  the  bean  plant  are 
typical  of  the  great  majority  of  living  things  in  that  they 
reproduce  by  means  of  two  cells  which  unite  to  form  a  ferti- 
lized egg-cell  able  to  develop  into  a  new  organism.  In  both 
animals  and  plants  the  egg-cells  are  very  similar  in  their 
microscopic  structure,  and  in  each  case  fertilization  causes 
the  egg-cell  to  begin  division  into  a  number  of  cells  which 
form  the  embryo.  The  cell  from  the  pollen-tube  which  fer- 
tilizes the  egg-cell  in  flowering  plants  (§  75)  corresponds 
to  the  sperm-cell  of  animals  (§  57).  In  some  flowerless 
plants  (e.g.,  sea- weeds)  there  are  motile  sperm-cells  which 
swim  in  water  just  as  those  of  fishes  or  frogs  do ;  but  these 
plants  live  where  water  surrounds  the  ovaries  producing  egg- 
cells  and  the  spermaries  producing  sperm-cells,  and  so  the 
sperm-cells  can  readily  swim  to  the  egg-cells  and  fertilize 
them.  The  absence  of  water  surrounding  the  flowers  of  the 
higher  plants  evidently  makes  such  a  method  of  sperm-cells 
reaching  egg-cells  impossible;  and  so  the  method  of  com- 
munication by  means  of  pollen-tubes  has  been  developed  as 
an  adaptation  to  the  peculiar  dry  conditions  under  which 
fertilization  must  take  place  in  most  flowers.  ,  However,  it 


132  APPLIED  BIOLOGY 

should  be  noted  that  the  important  point  concerning  fertiliza- 
tion is  that  it  is  the  union  of  two  cells  to  start  the  development 
of  an  embryo. 

The  method  of  reproduction  by  generative  cells  or  germ- 
cells  (sexual  reproduction),  which  has  been  illustrated  by  the 
frog  and  the  bean  plant,  is  the  only  method  in  a  large  number 
of  species  of  plants  and  animals ;  but  in  both  there  are  many 
species  which  may  at  times  reproduce  without  germ-cells. 
Reproduction  of  certain  plants  from  roots,  stems,  etc.,  will 
be  described  in  the  next  chapter ;  and  many  lower  animals 
reproduce  by  automatically  dividing  themselves.  Such  re- 
production of  plants  and  animals  without  germ-cells  is  termed 
asexual  (i.e.,  independent  of  sex). 


CHAPTER  VII 
CLASSIFICATION    OF   ANIMALS   AND   PLANTS 

123.    The  Scientific  Names  of  Animals  and  Plants.  —  The 

student  who  turns  over  many  pages  of  any  textbook  of 
botany  or  zoology  must  discover  that  the  scientific  names  are 
quite  different  from  the  common  ones.  Thus  our  ordinary 
garden  toad  is  in  zoological  terminology  Bufo  americanus, 
a  dog  is  Canis  familiaris,  a  house-cat  is  Felis  domestica,  a 
honey-bee  is  Apis  mellifica ;  and  so  for  all  the  known  animals 
there  is  such  a  double  name.  Likewise  among  plants,  the 
spring-beauty  is  Claytonia  virginica,  the  garden  beans  are 
Phaseolus  vulgaris,  pansy  is  Viola  tricolor,  blood-root  is 
Sanguinaria  canadensis,  and  the  liver-leaf  is  Hepatica  triloba. 
These  are  simply  examples,  taken  at  random,  of  names  of 
some  common  plants  and  animals. 

This  use  of  double  scientific  names  is  known  as  binomial 
nomenclature;  and  this  system  of  naming  was  introduced  by 
the  celebrated  Swedish  naturalist  Linnaeus,*  who  lived  be- 
tween 1707  and  1778.  It  has  a  number  of  advantages  over 
any  other  system  of  naming.  For  example,  the  pansy  is  only 
one  of  many  kinds  of  violets  which  are  known  ;  and  it  is  very 
convenient  to  have  the  word  Viola  for  all  kinds  of  violets, 
and  then  tricolor  to  distinguish  the  pansy  from  other  kinds  of 
violets,  each  of  which  has  its  own  special  name,  e.g.,  Viola 
rotundifolia  (round-leaved  violet),  Viola  odorata  (sweet 
violet),  Viola  lanceolata  (lance-leaved  violet),  and  so  on  for 
about  fifty  species  of  violets  which  grow  in  America. 

*  See  any  standard  encyclopedia  for  account  of  the  life  and  work  of 
Linnaeus. 

133 


134  APPLIED  BIOLOGY 

It  is  obvious  that  the  advantage  of  the  double  name  is 
the  same  as  in  a  human  family.  The  names  Smith,  Jones, 
etc.,  designate  families ;  but  we  must  have  a  second  name, 
such  as  John,  Thomas,  etc.,  to  indicate  individuals.  Like- 
wise in  biology,  names  like  Viola  serve  as  family  names  to 
include  all  closely  related  animals  or  plants,  and  a  second 
name  is  needed  to  distinguish  the  different  kinds  of  violets. 
In  writing  the  scientific  name  we  put  the  family  name  first ; 
but  we  also  do  the  same  thing  with  human  family  names 
when  making  an  alphabetical  list  of  names,  as  for  example  in 
a  biographical  dictionary  like  "  Who's  Who  in  America." 

But  why  should  scientific  names  not  be  in  English,  as  for 
example,  violet  sweet,  violet  yellow,  and  violet  lance-leaved, 
placing  the  family  name  in  English  form  first  and  adding  a 
second  name  to  distinguish  different  kinds  of  violets?  The 
answer  is  that  scientific  language  must  be  cosmopolitan  so 
that  students  in  any  language  can  understand ;  and  so  it  has 
been  agreed  to  base  scientific  names  on  the  classical  languages, 
making  the  names  Latin  in  form,  and  largely  of  Latin  and 
Greek  words.  Viola  tricolor  (violet  three-colored)  is  under- 
stood all  over  the  world  among  scientific  men  as  meaning 
the  plant  which  we  call  pansy,  which  some  people  call  "  heart's 
ease,"  and  for  which  there  are  other  local  names  in  many  of  the 
languages  of  Europe. 

Many  of  the  scientific  names  are  simply  the  Latin  name. 
For  example,  Bufo  americanus  is  Latin  for  American  toad, 
Phaseolus  vulgaris  for  common  bean,  Felis  for  cat,  and 
Canis  for  dog.  Sometimes  the  words  are  descriptive,  e.g., 
all  the  second  names  of  violets  mentioned  above,  Sanguinaria 
referring  to  the  juice  of  blood-root,  and  Hepatica  to  the  shape 
of  the  leaf  of  that  plant.  Some  names  are  in  honor  of  the 
discoverers  of  certain  plants  or  of  prominent  botanists ;  e.g., 
the  spring-beauty,  Claytonia,  was  named  for  Clayton,  an 
early  botanist;  and  Viola  brittoniana  is  a  species  of  violet 
named  for  Professor  Britton,  the  director  of  the  New  York 


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     135 

Botanical  Gardens.  Many  names  of  species  are  of  geographi- 
cal origin;  e.g.,  canadensis,  virginica,  americanus,  europea, 
etc. 

The  student  of  animals  and  plants  should  remember  that 
a  large  number  of  names  are  no  longer  descriptive.  For 
example,  Claytonia  virginica  is  found  in  many  states  besides 
Virginia ;  and  many  species  originally  named  vulgaris,  mean- 
ing common,  have  become  relatively  uncommon.  However, 
this  does  not  interfere  with  the  use  of  the  names  as  definite 
designations  for  particular  kinds  of  animals  and  plants. 
There  are  many  names  in  the  human  families  which  do  not 
correctly  describe  their  bearers,  e.g.,  Mr.  White,  a  negro; 
and  Mr.  Black,  a  white  man ;  but  these  meaningless  names 
serve  to  mark  the  individuals. 

124.  Genera  and  Species.  —  In  many  earlier  lessons  of 
this  book  the  word  species  has  been  used  in  its  popular  sense, 
meaning  a  kind  of  animal  or  plant.  It  is  now  time  for  a 
more  accurate  definition  of  the  word  as  used  in  biology. 

The  pansy,  Viola  tricolor,  has  been  referred  to  as  a  species 
of  violet.  One  might  buy  pansy  seed  from  hundreds  of 
dealers  in  seeds,  and  from  these  seeds  grow  thousands  of  in- 
dividual plants  which  by  the  form  of  stem  and  leaves,  and 
by  the  form  and  colors  of  the  flowers,  would  be  recognized 
by  any  gardener  as  pansy  plants.  In  short,  pansies  con- 
stitute a  species,  a  group  of  closely  similar  individuals. 

Now,  many  specimens  of  violet  plants  producing  yellow, 
blue,  and  white  flowers  might  be  collected ;  and  comparison 
would  show  that  the  individual  plants  with  yellow  flowers  are 
closely  similar,  but  differ  from  those  with  white  and  blue  flowers 
and  from  pansies.  Hence  the  similar  individual  plants  with 
yellow  flowers  would  be  named  as  a  species  distinct  from  the 
others.  Note,  however,  that  all  parts  of  the  plants  must  be 
compared,  and  not  color  alone,  as  the  above  illustration  might 
suggest.  For  example,  a  blue  violet  with  rounded  leaves 
would  be  considered  one  species,  and  a  blue  violet  with  lance- 


136  APPLIED  BIOLOGY 

shaped  leaves  would  be  considered  another  species.  Other 
points  of  structure,  such  as  shape  and  color  of  seeds,  size  of 
the  plant,  etc.,  enter  into  consideration;  and  so  there  are 
known  to  be  many  species  of  each  color  of  violet. 

All  the  species  of  violets  taken  together  constitute  the 
genus  Viola;  and  in  giving  the  double  names  to  species  the 
generic  name  Viola  is  followed  by  the  names  of  the  species,  e.g., 
Viola  tricolor. 

Summarizing  the  above,  we  may  define  a  species  as  a  group 
of  closely  similar  individuals;  and  a  genus  as  a  group  of 
similar  species.  As  an  illustration  of  an  animal  genus  and 
species,  we  may  take  the  dog  genus,  Canis,  which  includes 
dogs,  wolves,  and  jackals.  We  have  no  difficulty  in  recogniz- 
ing dogs,  because  they  have  certain  characteristics  which 
distinguish  them  from  wolves  and  jackals.  Hence  all  dogs 
are  included  in  the  one  species,  familiaris.  Likewise,  wolves 
differ  from  dogs  and  jackals,  and  so  wolves  are  classed  in 
other  species;  and  jackals  in  still  other  species. 

125.  Families,  Orders,  and  Classes.  —  Genera  (plural  of 
genus)  are  grouped  into  families.  For  example,  fox,  wolf, 
dog,  and  other  dog-like  mammals  are  grouped  together  in  the 
dog  family,  Canidae.  Similarly,  cat,  lion,  tiger,  leopard, 
lynx,  and  many  other  cat-like  animals,  belonging  to  several 
genera,  are  grouped  together  in  the  cat  family,  Felidse.  The 
marten,  mink,  otter,  weasel,  skunk,  and  badger  belong  to 
a  related  family,  the  marten  family.  Then  there  is  a  bear 
family,  and  a  raccoon  family.  All  the  animals  of  the  dog,  cat, 
marten,  bear,  and  raccoon  families  have  some  general  resem- 
blances, particularly  in  that  their  structure  is  adapted  to  a 
flesh  diet,  hence  they  are  called  the  flesh-eaters  or  carni- 
vores. Such  an  assemblage  of  similar  families  constitutes 
an  order. 

Another  familiar  order  is  that  of  the  hoofed  animals,  such 
as  cow,  horse,  pig,  sheep.  Another  is  that  of  the  gnawers  or 
rodents  (rats,  mice,  squirrels,  rabbits,  gophers).  Another 


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     137 

order  contains  the  opossum  and  kangaroo,  animals  with  a 
pouch  for  carrying  the  young.  Now,  all  these  animals  have 
certain  common  characteristics.  For  example,  a  cow,  dog, 
rat,  and  kangaroo  all  have  hair,  all  feed  their  young  with 
milk,  and  all  have  a  diaphragm  for  breathing.  None  of  these 
important  things  is  found  in  birds,  frogs,  fishes,  worms,  and 
other  low  animals.  Hence  the  animals  which  have  these 
characteristics  are  considered  related  in  one  large  group. 
And  to  such  a  group  of  orders  the  name  class  is  applied ;  and 
the  particular  class  to  which  the  flesh-eaters,  the  gnawers, 
the  hoofed  animals,  and  several  other  orders  belong  is  Mam- 
malia, or  mammals,  a  name  referring  to  the  fact  that  the 
young  are  fed  with  the  secretion  of  milk-glands  or  mammary 
glands. 

126.  Phyla,  or  Divisions.  —  Classes  in  turn  are  united 
into  larger  groups.  The  class  Mammalia  and  the  classes 
to  which  birds,  reptiles,  frogs  and  fishes  belong  are  all  com- 
posed of  animals  which  have  a  backbone  or  vertebral  column. 
This  larger  group,  which  contains  all  animals  with  a  back- 
bone (fishes,  amphibia,  reptiles,  birds,  and  mammals),  is  one 
of  the  great  divisions  of  the  animal  kingdom,  and  has  long 
been  known  as  the  phylum  or  division  of  vertebrates  (Verte- 
brata,  but  some  authors  prefer  Chordata). 

Other  primary  divisions  or  phyla  (plural  of  phylum)  of  the 
animal  kingdom  are  those  represented  by  the  shelled  animals 
(oysters  and  snails),  by  the  animals  with  jointed  legs  (cray- 
fish, lobster,  crabs,  spiders,  insects),  by  the  worm-like  ani- 
mals (earthworm,  tapeworm,  leech,  etc.),  by  the  jelly-like 
animals  (jellyfish,  coral-animal),  by  the  sponge-animals,  and 
by  the  simplest  microscopic  animals.  The  technical  names 
are  given  in  §  133,  but  for  our  present  purpose  it  is  sufficient 
to  state  that  there  are  about  a  dozen  such  primary  groups  or 
phyla  in  the  animal  kingdom.  Each  such  primary  division 
may  be  called  a  division,  phylum,  or  branch  ;  but  phylum  is 
used  by  most  zoologists. 


138  APPLIED  BIOLOGY 

127.  Summary  of  Animal  Classification.  —  Reversing  the 
above  order  of  presentation,  the  animal  kingdom  is  composed 
of  divisions  or  phyla,  each  phylum  01  classes,  a  class  of  orders, 
an  order  of  families,  a  family  of  genera,  a  genus  of  species,  a 
species  of  individuals.     And  sometimes  there  are  varieties 
of  individuals  in  a  species ;   e.g.,  various  well-known  breeds 
or  varieties  of  dogs  in  the  dog  species. 

The  full  classification  of  a  bulldog  is  as  follows  :  Kingdom, 
animal ;  phylum  or  division,  Vertebrata  (vertebrates) ;  class, 
Mammalia  (mammals) ;  order,  Ferae  (carnivores) ;  family, 
Canidse  (dog  family) ;  genus,  Canis ;  species,  familiaris ; 
variety,  bulldog ;  and  individual  dogs,  which  are  not  exactly 
alike.  j 

128.  Summary  of  Plant  Classification. — In  a  similar  way 
botanists  have  classified  plants  into  groups.     The  primary 
divisions  of  the  plant  kingdom  are  into  lowest  plants,  moss- 
like   plants,    fern-like   plants,    and   seed-plants.     (For   the 
technical  names  see  §  133.)     Taking  the  division  of  seed- 
plants,  it  is  subdivided  (§  209)  into  gymnosperms  (flowers 
in  form  of  cones,  like   pines)  and  angiosperms  (with  true 
flowers).     Taking  the  higher  group,  the  angiosperms,  it  is 
subdivided  into  the  classes  of  monocotyledons  and  dicotyle- 
dons (§  141).     Each  of  these  classes  is  composed  of  orders, 
which  are  subdivided  into  families  (e.g.,  composite  family, 
§  207),  families  into  genera  (e.g.,  Viola,  §  124),  genera  into 
species,  and  sometimes  species  into  varieties  (see  long  lists 
of  varieties  of  pansy  species  in  seed-catalogues).     It  is  evi- 
dent that  the  general  plan  of  classification  is  essentially  the 
same  for  both  animals  and  plants. 

129.  Classification  Based  on  Resemblances.  —  It  is  evi- 
dent from  the  foregoing  illustrations  that  the  classification 
of  animals  and  plants  is  based  on  the  fact  that   there  are 
certain  resemblances  in  structure.     Thus  all  vertebrates  are 
alike  in  having  a  backbone  and  some  other  structures  not 
found  in  lower  animals ;  all  mammals  are  alike  in  having  not 


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     139 

only  a  backbone,  but  also  hair,  milk-organs,  and  diaphragm ; 
all  carnivores  are  alike  in  having  mouth  and  digestive  organs 
t  adapted  to  meat-eating  in  a  way  not  found  in  other  mammals ; 
and  dogs  and  wolves  are  nearer  alike  in  all  respects  than 
they  are  like  any  other  animals.  It  is  all  a  question  of  re- 
semblances— resemblances  in  detail  between  individuals  of 
a  species,  and  more  and  more  general  resemblances  as  we  pass 
from  species  to  larger  and  larger  groups  (genera,  orders, 
classes),  until  we  find  in  divisions  or  phyla  only  such  general 
resemblances  as  the  backbone,  which  characterizes  vertebrates. 
These  resemblances,  or  points  of  structure  which  mark  the 
animals  or  plants  belonging  to  any  particular  group,  are  the 
characteristics.  For  example,  the  backbone  is  a  characteristic 
of  vertebrates,  for  no  lower  animals  have  it ;  and  hair,  milk- 
glands,  and  the  diaphragm  are  characteristics  of  mammals; 
the  formation  of  seeds  in  a  peculiar  manner  is  a  characteristic 
of  seed-plants ;  the  arrangement  of  flowers  in  a  peculiar  head 
is  a  characteristic  of  composites.  So  for  each  large  or  small 
group  of  animals  and  plants,  from  species  to  primary  divisions, 
there  are  distinguishing  marks  based  on  resemblances. 

130.  The  Reasons  for  Classification.  —  Probably  the  first 
reason  why  men  of  science  have  classified  animals  and  plants 
is  that  it  has  seemed  natural  to  group  together  the  similar 
things  in  nature.  The  fact  is  that  we  seem  naturally  inclined 
to  associate  together  things  which  are  alike,  and  thereby 
distinguish  between  the  unlike.  Without  any  idea  of  classify- 
ing in  a  scientific  way,  we  are  constantly  noting  the  character- 
istics of  people  we  see;  and  we  compare  and  distinguish 
between  people  of  different  races  (white,  black,  red,  etc.),  of 
nations  (German,  English,  French,  Swedish,  etc.),  of  people 
from  different  regions  of  a  country  (Northern,  Southern,  and 
Western  in  United  States),  and  of  individuals  in  families. 
If  we  see  two  or  more  strangers  who  look  alike,  we  think  of 
them  as  closely  related,  and  in  the  same  family.  If  we  see 
that  the  members  of  several  families  have  some  general  simi- 


140  APPLIED  BIOLOGY 

larity  of  form,  manners,  language,  etc.,  we  naturally  classify 
the  families  as  members  of  the  same  nation.  And  we  think 
of  Germans,  French,  English,  and  Swedish  nations  as  belong- 
ing to  the  white  race ;  and  we  think  of  Chinese  and  Japanese 
as  belonging  to  a  different  race.  Also  we  classify  men  accord- 
ing to  their  political  and  religious  beliefs  and  their  business 
in  life.  When  we  stop  to  consider  the  matter,  it  becomes 
evident  that  we  are  all  the  time  classifying  things  which  in- 
terest us  in  everyday  life. 

Now,  naturalists  (biologists)  have  followed  the  same  tend- 
ency in  grouping  similar  animals  and  plants.  Noting  that 
dogs  and  wolves  and  foxes  are  very  much  alike,  naturalists 
have  long  regarded  these  animals  as  in  one  family,  just  as  we 
think  of  two  similar  human  individuals  as  in  the  same  family. 
And  then,  noting  that  the  members  of  other  families  of  ani- 
mals (e.g.,  bear  and  raccoon)  have  some  of  the  same  char- 
acteristics, naturalists  have  united  the  families  into  an  order, 
just  as  we  unite  similar  human  families  into  nations.  And 
orders  are  united  into  classes,  as  nations  into  races  of  men. 
In  some  such  way  naturalists  have  been  at  work  from  before 
the  days  of  Aristotle  (B.C.,  384-322),  who  wrote  the  first 
books  on  animals  and  plants,  and  they  have  gradually  grouped 
similar  forms  together  into  species,  genera,  families,  orders, 
and  larger  groups.  Many  of  these  groups  are  not  yet  com- 
plete, for  many  animals  and  plants  have  not  yet  been  studied 
carefully  enough  to  show  what  other  species  they  most  closely 
resemble. 

Another  reason  for  classification  is  that  it  is  convenient  for 
references,  just  as  in  a  library  it  is  useful  to  have  all  the  books 
on  a  given  subject  placed  together.  Since  there  are  several 
hundred  thousand  species  of  animals  and  plants,  it  would 
be  quite  impossible  to  find  the  descriptions  and  names  if  we 
had  no  classification.  But  with  classification  identification 
is  easy.  A  new  animal  was  found  in  Africa  recently.  It 
had  a  backbone,  and  was  therefore  a  vertebrate.  It  had 


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     141 

hair  and  other  structures,  which  showed  that  it  was, a  mam- 
mal. It  had  hoofs  and  other  structures  similar  to  the  cow, 
deer,  etc.,  and  it  therefore  belonged  to  the  order  of  hoofed 
animals.  And  finally  it  had  close  similarity  to  members  of 
the  antelope  family.  Closer  comparison  with  the  named 
species  of  antelopes,  of  which  descriptions  have  been  published 
and  specimens  stored  in  the  great  museums  of  the  world, 
will  make  it  possible  to  determine  the  genus  to  which  this 
new  specimen  belongs.  And  then  the  naturalist  who  writes 
a  description  of  the  specimen  will  give  a  name  to  the  species, 
so  that  the  next  man  who  looks  for  the  name  of  this  animal 
can  find  it  in  the  proper  place  in  the  scheme  of  classification. 
Such,  in  essentials,  is  the  story  of  the  use  of  systems  of 
animal  and  plant  classification  whenever  any  one  wants  to  give 
a  name  to  a  new  specimen  or  to  find  out  whether  it  has  been 
given  a  name.  It  is  simply  necessary  to  begin  with  the  charac- 
teristics of  the  larger  divisions,  and  then  work  down  through 
classes,  orders,  families,  genera,  to  species ;  and  since  the  great 
books  on  classification  (e.g.,  Gray's  "  Manual  of  Botany," 
or  Jordan's  "  Manual  of  Vertebrates")  are  arranged  in  this 
order,  it  is  easy  for  one  who  carefully  observes  and  compares 
to  trace  out  the  name  of  a  plant  or  animal  specimen.  This 
practical  use  of  classification  is  made  every  day  by  numerous 
people  who  seek  the  names  of  organisms  for  the  pleasure  of 
feeling  acquainted  with  them,  or  by  biologists  whose  pro- 
fessional work  makes  it  necessary  for  them  to  find  the  names 
of  their  specimens. 

131.  Practical  Work  in  Classification.  —  (D)  A  small  collection  of 
crayfishes,  shrimps,  lobster,  and  various  species  of  crabs  may  be 
used  to  illustrate  some  of  the  chief  principles  stated  above  regarding 
species,  genera,  and  larger  groups.  A  collection  of  insects  in  alcohol 
is  especially  excellent  for  the  same  purpose.  The  specimens  of 
a  mixed  collection  should  be  arranged  in  groups,  according  to 
obvious  similarity  to  grasshopper,  butterfly,  beetle,  dragon-fly,  and 
house-fly.  This  exercise  can  be  made  to  illustrate  the  leading  prin- 
ciples of  classification  without  going  into  detailed  study  of  the  in- 


142  APPLIED  BIOLOGY 

sects  at  this  time.  Later,  after  study  of  insects  in  Chapter  XIV, 
the  students  should  have  practice  in  identifying  common  insects 
with  reference  to  the  orders  represented  by  those  named  above. 
For  the  present  purpose  it  will  be  sufficient  to  speak  of  grasshopper 
group,  beetle  group,  butterfly  group,  crayfish  group,  crab  group,  etc., 
leaving  the  technical  names  of  these  until  later  lessons. 

For  plant  classification,  a  set  of  leaves,  fruits,  branches,  bark, 
etc.,  of  two  or  more  species  of  some  common  plants,  e.g.,  oaks, 
maples,  or  others  available.  In  the  spring  of  the  year,  members  of 
the  rose  family  (apple,  pear,  peach,  plum,  cherry,  quince,  wild  rose, 
etc.)  will  furnish  some  interesting  materials  for  classification ;  and 
in  the  autumn  there  are  many  composites  which  might  be  used  in 
the  same  way. 


132.  Learning  Scientific  Names.  —  It  is  not  necessary  that 
all  the  technical  names  given  in  the  following  tables  of  classi- 
fication be  memorized  at  one  time.  The  best  way  to  learn 
them  is  through  use.  Only  the  more  important  names  have 
been  given,  and  many  of  them  are  rapidly  becoming  com- 
mon terms.  In  fact,  few  people  realize  that  many  common 
names  are  also  the  scientific  ones.  For  example,  among 
common  plants  grown  in  home  gardens  :  asparagus,  spinacia 
(spinach),  aster,  alyssum,  calendula,  centaurea  (sometimes 
called  bachelor's  button),  chrysanthemum,  phlox,  crocus, 
cosmos,  iris,  smilax,  narcissus  (sometimes  called  daffodil), 
tulipa  (tulip),  hyacinthus  (hyacinth),  yucca,  lilium  (lily), 
viola  (violet),  petunia,  portulaca,  salvia,  thymus  (thyme), 
zinnia,  verbena,  heliotropum  (heliotrope),  —  all  these  and 
many  more  are  the  scientific  names  of  the  genus  in  each 
case;  but  since  for  most  of  these  plants  there  is  no  common 
name,  the  scientific  name  has  become  also  the  common  one. 
This  use  of  scientific  as  common  names  should  be  encouraged 
whenever  there  is  confusion.  Such  a  case  is  that  of  "  mari- 
gold," which  has  been  applied  to  so  many  yellow-flowered 
composites  that  even  gardeners  who  have  not  studied  botany 
are  learning  to  use  the  scientific  names  calendula,  coreopsis, 
and  tagetes  as  common  names.  Seed-dealers  are  helping 


CLASSIFICATION  OF  ANIMALS  AND  PLANTS     143 

popularize  such  names  by  putting  the  scientific  names  on 
packages  of  seeds. 

It  is  evident  from  the  above  list  of  scientific  names  made 
common  by  popular  usage  that  there  is  no  truth  in  the  state- 
ment that  scientific  names  are  hard  to  learn.  Any  one  who 
can  learn  such  a  name  as  chrysanthemum  can  surely  learn  most 
other  scientific  names  when  they  are  needed.  And  in  order 
to  be  definite,  they  are  often  needed.  Therefore  the  student 
should  drop  all  prejudice  arising  from  the  supposed  difficulty 
of  scientific  names,  and  aim  to  make  frequent  use  of  the  most 
common  ones. 

133.  Chief  Group  of  Animals  and  Plants.  —  The  tables 
below  are  intended  to  show  the  most  important  of  the  pri- 
mary subdivisions  of  the  animal  and  plant  kingdoms.  The 
lowest  group  is  named  first  and  the  highest  last,  but  in  the  case 
of  the  animals  there  is  doubt  as  to  the  relative  position  of  some 
of  the  intermediate  groups  ;  for  example,  whether  annelids  are 
higher  than  echinoderms,  or  arthropods  higher  than  mollusks. 

ANIMALS 

(A  primary  group  of  animals  usually  called  a  phylum,  plural  phyla.) 

Phylum        I.   Protozoa  (simplest,  one-celled  animals). 
Phylum      II.  /-Porifera  (sponge-animals). 

Phylum     III.-' Coelenterata  (hydroids,  jellyfishes,  coral-animals). 
Phylum     IV.   Platyhelminthes  (flat  worms,  tapeworms). 
Phylum       V.    Nemathelminthes  (round  worms). 
Phylum     VI X  Annelida  (segmented  worms). 
Phylum    VII./Echinoderma   (starfish,  sea-urchin,   crinoid,   sea- 
cucumber)' 

Phylum  VIII.  Arthropoda  (lobster,  crab,  spider,  insects). 
Phylum  IX./Mollusca  (clam,  oyster,  snail,  cuttle-fish). 
Phylum  X.  Vertebrata  (backboned  animals). 

The  large  textbooks  of  zoology  include  several  other  phyla 
of  animals  not  belonging  to  any  of  the  ten  groups  named 
above.  Most  of  these  are  small  marine  worm-like  animals 


144 


APPLIED  BIOLOGY 


which  few  except  zoologists  have  occasion  to  examine.  The 
most  common  animals  belong  in  the  ten  phyla  named  above. 

The  subdivision  of  most  of  the  above  phyla  into  classes 
is  given  at  ends  of  the  chapters  in  Part  III  of  this  book. 

Phyla  I-IX  inclusive  are  often  called  Invertebrates  or 
backboneless  animals. 


PLANTS 

(There  are  four  grand  divisions  or  primary  groups  of  plants). 

Division       I.    Thallophyta  (simplest  plants). 

(1)  Algss    (with    chlorophyll).     Examples :    sea- 

weeds, many  minute  aquatic  plants. 

(2)  Fungi     (without     chlorophyll).      Examples : 

molds,  mushrooms. 

Division     II.   Bryophyta.     Examples :    liverworts  and  mosses. 
Division  III.    Pteridophyta.     Examples  :  ferns,  horsetails,  lyco- 

pods. 
Division    IV.   Spermaphyta  (or  Phanerogamia) 

(1)  Gymnospermae.     Examples  :  cycads  and  coni- 

fers. 

(2)  Angiospermse. 

(a)  Monocotyledones.       Examples :      palms, 

grasses,  lilies,  orchids. 
(6)  Dicotyledones.     Examples :  most  of  the 

common  trees,  vegetables,  and  "flowers." 

The  plants  in  Divisions  I,  II,  III,  are  often  termed  "Spore- 
Plants,"  "Flowerless  Plants,"  or  "  Cryptogams  "  ;  while  those  of  IV 
are  "Seed-Plants"  or  "Flowering-Plants." 


PART   II 

PRINCIPLES  OF  BIOLOGY  ILLUSTRATED  BY 
TYPES   OF   PLANTS* 

PART  I  has  given  an  introduction  to  many  of  the  general 
facts  and  principles  concerning  animals  and  plants.  It  has 
been  possible  to  do  this  by  studies  of  one  animal  and  one  plant, 
because  in  a  general  way  there  is  great  similarity  among 
living  things.  But  the  introductory  study  will  lose  much  of 
its  value  as  useful  science  unless  the  reader  goes  on  to  study 
illustrations  and  applications  of  the  general  principles. 
Especially  would  one  who  had  studied  but  two  living  things 
have  a  very  limited  knowledge  of  the  various  forms  of  animal 
and  plant  life,  and  of  the  many  remarkable  modifications  of 
structure  of  organisms  which  adapt  them  for  carrying  on  the 
life-processes  in  various  ways.  In  order  to  extend  our  knowl- 
edge of  the  principles  of  plant  biology,  we  shall  now  make 
a  series  of  comparative  studies  of  seed-plants  or  flowering- 
plants,  and  later  we  shall  study  some  of  the  most  common 
flowerless  plants  (moss,  fern,  mushroom,  etc.). 

*  To  THE  TEACHER  :  For  possible  changes  in  order  of  study  of  this  and 
later  chapters  see  suggestions  in  the  preface  of  this  book  and  especially 
Chapter  VIII  of  the  "Teachers'  Manual  of  Biology,"  which  accompanies 
this  book. 


145 


CHAPTER  VIII 
STUDIES    OF   SEED-PLANTS 

134.  This  chapter  will  direct  the  study  of  the  parts  of  seed- 
plants  which  are  most  modified  in  adaptation  to  various 
habits  of  life.     Since  many  seed-plants  are  peculiar  only  with 
regard  to  some  one  organ,  it  is  most  interesting  to  study  and 
compare  corresponding  parts  of  various  plants  rather  than 
compare  entire  plants  with  each  other.     Accordingly,  we 
shall  study  (1)  types  of  seeds  and  their  germination,  (2)  roots, 
(3)  stems,  (4)  leaves,  (5)  flowers,  (6)  fruits  —  in  each  case 
giving  attention  to  special  uses  of  these  organs  in  the  life- 
activities  of  the  plants  which  possess  them.     Prominence  is 
given  to  the  seed-plants  because  they  are  so  common  and  so 
important  economically. 

TYPES  OF  SEEDS  AND   SEEDLINGS 

135.  The  bean  seed  has  served  as  an  introduction  to  the 
structure  and  germination  or  awakening  of  seeds ;  but  with- 
out  examination  of   such  seeds   as   pea,  squash,  castor-oil 
plant  and  corn  it  will  be  difficult  to  understand  the  peculiari- 
ties of  germination  which  one  may  see  in  any  garden  where 
many  different  kinds  of  plants  are  grown.     In  §  140  is  given 
a  special    list    of    seeds   for   optional    study    by   members 
of  the  class  who  work  rapidly  and  at  the  same  time  care- 
fully, or  who  are  in  advance  of  the  class  because  some  of 
the  germination  studies  are  reviews  of  nature-study  lessons 
taken  in  the  elementary  schools. 

136.  Pea  Seed.  —  (L)   Materials :    dry  peas,  both  smooth  and 
wrinkled  varieties ;    some  soaked  in  water  24  hours ;    seedlings  of 

146 


STUDIES    OF   SEED-PLANTS  147 

various  stages  grown  on  moist  paper  (§  81),  which  may  be  preserved 
for  years  in  5  per  cent  formalin  solution;  some  peas  planted  one 
week,  and  some  two  weeks  before  the  lesson,  some  in  soil  in  pots  and 
some  in  a  tumbler  or  lamp-chimney  arranged  as  described  in  §  81 ; 
and  some  seedlings  with  hypocotyl  marked  with  waterproof  ink  to 
determine  the  region  of  most  rapid  growth  (see  Fig.  34,  bean). 

In  connection  with  this  lesson  on  seeds  consult  figures  in  Atkinson's 
"First  Studies  of  Plant  Life." 

Examine  pea  seeds,  following  the  directions  given  for  the  bean 
(§  79).  Examine  seedlings  of  various  stages  and  compare  with  those 
of  the  bean.  Especially  compare  the  ways  of  the  two  plants  in 
getting  out  of  the  soil.  Make  sketches  showing  structure  of  the 
pea  seed  and  stages  in  its  germination.  Write  in  your  note-book  a 
short  paragraph  comparing  the  germination  of  pea  with  bean  seeds. 

Recall  that  the  stem  part  of. the  hypocotyl  (that  is,  the  stem 
between  root  and  cotyledons)  elongated  rapidly  during  germination 
of  bean  seeds  and  pushed  the  cotyledons  out  of  the  soil  (consult 
your  labeled  drawings) ;  and  then  try  to  explain  why  the  cotyledons 
of  the  pea  seed  remain  where  they  are  planted  in  the  ground.  Ex- 
amine specimens  of  pea  seedlings  whose  hypocotyls  have  been 
marked  with  waterproof  ink  (see  Fig.  34,  §  81),  and  determine  whether 
the  root  has  been  growing  downward,  or  the  stem  just  below  the 
cotyledons  elongating  rapidly,  or  both.  Make  sketches  and  compare 
with  corresponding  sketches  of  marked  bean  seedlings. 

137.  Squash  Seed.  —  (L)  Materials  :  dry  squash  seeds ;  seeds 
soaked  in  warm  water  for  12  hours ;  seeds  soaked  for  24  hours ;  seeds 
in  various  stages  of  germination  (may  be  preserved  in  formalin) ; 
seeds  planted  in  soil  two  or  three  weeks  in  advance  of  lesson ;  and 
also  seeds  placed  in  various  positions  in  a  lamp-chimney  or  tumbler 
as  in  §  81,  so  that  the  students  can  follow  the  changes  from  day  to 
day. 

Examine  first  a  dry  seed,  and  notice  the  color  and  texture  of  its 
covering  (seed-coat,  or  testa).  At  the  pointed  end  is  the  hilum  and 
inside  this  is  a  small  hole,  the  micropyle  —  this  is  best  seen  in  a 
soaked  seed. 

Open  a  soaked  squash  seed  (this  can  easily  be  done  by  first  cutting 
around  the  edge  of  the  flattened  seed,  and  then  inserting  the  knife 
and  prying  it  open) .  Remove  the  green  covering  from  the  "kernel," 
and  examine  the  embryo.  At  the  small  pointed  end  is  the  hypocotyl. 
Separate  the  two  cotyledons  from  each  other  and  note  that  they  are 
fastened  together  only  at  the  hypocotyl.  Follow  development  of 
squash  seeds  planted  in  soil,  in  tumbler  as  in  §  81,  and  laid  on  paper 


148 


APPLIED    BIOLOGY 


kept  moist.  Notice  a  peculiar  hump  (the  peg)  which  develops  on 
the  hypocotyl  and  when  that  grows  longer  the  seed-coats  are  pried 
open.  Notice  how  the  stem  part  of  the  hypocotyl  is  looped  as  it 
comes  from  the  seed-coat.  In  specimens  planted  in  the  ground  this 
loop  is  the  first  part  to  appear,  soon  dragging  the  cotyledons  out 
after  it,  and  then  the  looped  stem  straightens  out  and  lifts  up  the 
two  green  cotyledons. 

Compare  cotyledons  of  squash  and  bean  seedlings  at  various  stages 
until  the  second  pair  of  leaves  develop,  and  write  a  brief  account. 

Take  a  squash  seed  that  has  sprouted,  and  open  as  before.  Be- 
tween the  cotyledons,  and  fastened  to  the  upper  end  of  the  hypocotyl, 
is  the  epicotyl,  which  is  too  small  to  be  easily  seen  in  a  dry  seed 
and  one  must  look  for  it  after  it  has  grown  larger.  Examine  the 
epicotyl  in  seedlings  of  various  sizes. 

Compare  a  squash  seed  with  a  bean  seed  in  the  following  points : 
texture  of  seed-coat,  number  and  thickness  of  cotyledons, 
the  more  leaf-like  cotyledons,  and  relative  size  of  epicotyl.  Record 
these  facts  in  tabular  form. 

.    138.  Seed    of    Castor-Oil    Plant.  —  (L)   Materials :    dry   seeds 
(poisonous).     Seeds  soaked  in  water  for  at  least  24  hours.     A  series 

of  sprouted  specimens  to  show  the  de- 
velopment of  the  seedling  up  to  the 
time  the  epicotyl  is  well  developed. 

Examine  dry  castor-oil  seeds  and 
note  color  and  markings.  The  meaning 
or  use  of  these  is  not  known.  In  fact, 
it  is  not  necessary  to  assume  that  they 
have  a  use ;  for  hundreds  of  beautiful 
minerals  are  buried,  and  therefore  their 
color  as  seen  in  the  light  cannot  be  of 
use.  Some  authors  have  imagined  that 
castor-oil  seeds  resemble  beetles  or  ticks 
so  closely  that  they  would  be  avoided 
by  birds,  but  there  is  no  scientific  evidence  to  support  this  theory. 
Examine  a  castor-oil  seed  which  has  been  soaked  in  water.  The 
point  of  attachment  in  the  ovary  (that  is,  the  hilum)  and  the  mi- 
cropyle  are  near  the  rounded  projection  at  the  small  end  of  the  seed. 
Crack  the  shell-like  seed-coat,  and  carefully  remove  it.  The  "ker- 
nel" thus  exposed  is  covered  with  a  delicate  membrane  (inner  seed- 
coat).  Carefully  remove  this.  The  double  "kernel"  will  at  first 
be  taken  to  correspond  to  the  embryo  of  a  bean  with  very  thick 
cotyledons;  but  by  carefully  separating  the  two  halves  of  water- 


FIG.  42.  Castor-oil  seed,  c,  hy- 
pocotyl; st,  cotyledon;  e,  en- 
dosperm. (From  Osterhout.) 


STUDIES    OF    SEED-PLANTS  149 

soaked  seeds,  and  especially  some  which  have  begun  to  germinate, 
it  will  be  found  that  most  of  the  two  halves  taken  to  be  the  coty- 
ledons like  those  of  the  bean  are  masses  of  stored  food  (really  nu- 
merous cells  stored  with  food)  and  that  between  them,  in  the  center  of 
the  seed,  are  two  delicate  white  cotyledons  joined  to  the  hypocotyl, 
which  is  at  the  hilum  end  of  the  seed.  Carefully  separate  the  coty- 
ledons from  the  food-masses  (called  endosperm),  taking  care  not 
to  break  off  the  hypocotyl.  The  embryo  will  thus  be  freed  from 
all  other  parts  of  the  seed.  Examine  the  parts  of  the  embryo, 
especially  in  seeds  which  have  begun  to  sprout.  Notice  the 
epicotyl. 

Examine  a  series  of  seedlings  from  castor-oil  seeds  planted  in 
soil ;  some  placed  in  a  tumbler  as  in  §  81 ;  and  some  kept  on  moist 
paper.  Find  out  how  the  embryo  gets  out  of  the  seed-coat ;  how 
it  comes  out  of  the  ground ;  what  becomes  of  the  endosperm  ;  why 
the  hypocotyl  remains  bent  so  long ;  development  of  the  cotyledons ; 
compare  the  seedlings  with  those  of  bean  and  squash  previously 
studied.  Make  a  series  of  sketches  showing  stages  in  germination 
of  castor-oil  seed. 

139.  Corn  Grain.  —  (L)  NOTE  :  By  corn  is  here  meant  Indian 
corn  or  maize.  In  the  United  States  the  word  is  always  so  under- 
stood ;  but  in  other  countries  the  word  is  used  in  its  original  sense, 
meaning  grains  of  any  cereals  —  corn,  wheat,  oats,  barley,  rye.  It 
is  called  Indian  corn  because  found  under  cultivation  by  the  Indians 
when  America  was  discovered  by  Columbus.  The  name  maize  is 
from  the  Indian  name,  and  the  scientific  name  is  Zea  Mays. 

Materials  :  dried  grains  or  kernels  of  sweet,  "field,"  and  pop- 
corn; dried  ears  of  corn;  "green"  corn  on  the  cob  (fresh  or  in 
formalin  solution) ;  some  ears  with  husks ;  very  young  ears  showing 
the  attachment  of  the  silk;  grains  of  yellow  "field"  corn  soaked  in 
water  for  several  days ;  sprouted  grains  in  all  stages  of  development 
(may  be  preserved  in  formalin) . 

Examine  specimens  of  ears  of  corn  with  and  without  the  husks 
removed.  The  central  axis  or  cob  is  really  the  mature  form  of  a  sort 
of  flower-stalk  on  which  there  were  as  many  flowers  as  there  are 
grains  on  the  ear.  Each  individual  flower  which  forms  a  grain  has 
only  the  pistil,  the  petals  and  sepals  being  absent  and  the  stamens 
in  separate  flowers  in  the  tassels  at  top  of  the  corn  plant.  Since  each 
pistil  develops  into  a  grain,  this  is  therefore  a  fruit  in  the  botanical 
sense  (§  212) ;  and  each  flower  produces  one  fruit  containing  one 
embryo.  In  the  bean  each  flower  produces  one  fruit  (the  pod),  but 
this  fruit  happens  to  have  several  seeds,  each  with  an  embryo,  that 


150  APPLIED    BIOLOGY 

is,  one  bean  flower  produces  several  embryos.  A  bean  pod  with  one 
seed  would  be  equivalent  to  a  grain  of  corn  because  it  would  be  one 
seed  produced  by  one  pistil. 

Examine  arrangement  of  grains  on  the  cob.  The  attachment  of 
each  grain  corresponds  to  the  flower-stalk  or  pedicel  of  the  bean- 
pod.  At  the  opposite  (free)  end  of  the  grain  is  the  withered  base 
of  the  style,  best  seen  as  a  sharp  point  in  some  varieties  of  pop- 
corn. In  immature  ears  it  is  easy  to  see  that  a  thread  of  so-called 
"corn  silk"  (each  thread  is  really  a  greatly  elongated  style)  is  at- 
tached to  the  free  end  of  each  grain.  The  "silk"  extends  from  each 
grain  in  the  groove  between  two  rows  of  grains  to  the  end  of  the  cob 
where  the  surrounding  "leaves"  or  "husks"  are  arranged  so  that  the 
ends  of  the  "silks "  are  exposed.  These  exposed  ends  are  the  stigmas, 
one  for  each  flower  (represented  by  a  pistil,  and  in  the  mature  ear 
by  a  grain).  Pollen-dust  from  the  tassels  (which  are  flowers  with 
stamens  only)  falls  or  is  blown  by  the  wind  upon  the  exposed  "silks" 
or  stigmas.  Down  each  "silk"  or  style  a  pollen- tube  grows  to  the 
ovule  in  the  ovary.  Then  the  ovary  with  its  ovule  develops  into 
a  grain  of  corn  with  an  embryo  corn  plant. 

Examine  grains  of  field,  sweet,  and  pop-corn  and  notice  on  one  of 
the  flat  sides  of  each  grain  an  oval  patch  (usually  a  groove  in  dry 
grains)  which  marks  the  position  of  the  embryo  beneath  the  covering 
of  the  grain.  Look  at  an  ear  of  sharp-pointed  pop-corn  and  deter- 
mine whether  the  embryo  is  on  the  side  toward  the  stalk  or  toward  the 
free  end  of  the  ear,  and  also  note  in  which  direction  the  remains  of 
the  style  is  turned.  Do  you  see  any  reason  for  this  in  reference  to 
position  of  the  "silks"  ?  Now  look  at  an  ear  of  "field"  corn  and 
compare.  The  style  is  not  so  prominent  in  this  variety,  but  it  is  a 
small  elevation  between  embryo  and  end  of  grain. 

Imagine  an  ear  of  corn  split  longitudinally,  and  make  a  diagram 
showing  relation  of  grains  to  cob,  and  also  indicate  by  dotted  lines 
the  position  of  "silks"  extending  from  a  few  grains. 

Remove  the  covering  from  a  water-soaked  grain  or  from  "green" 
corn  (fresh  or  preserved  in  formalin).  This  covering  is  the  "hull" 
which  is  removed  when  making  hominy  or  samp.  In  the  old- 
fashioned  method  of  preparing  hominy  the  grains  were  first  boiled 
in  lye  from  wood  ashes  which  loosened  the  hull,  and  then  soaked  in 
water  to  remove  the  lye. 

With  the  sharp  point  of  a  knife,  lift  out  the  corn  embryo  (some- 
times called  the  "germ").  The  remainder  of  the  grain  (endosperm) 
is  food  for  the  embryo.  In  the  yellow  varieties  of  corn  the  endo- 
sperm is  yellow  and  the  embryo  whitish  in  color. 


STUDIES    OF    SEED-PLANTS  151 

In  order  to  identify  the  parts  of  the  embryo,  it  is  best  co  examine 
first  some  stages  in  germination. 

Examine  a  corn  seedling  just  coming  out  of  the  soil.  Only  the 
epicotyl  appears  as  a  coil  of  leaves  rolled  up  with  the  younger  leaves 
within.  Dig  up  such  a  seedling,  or  study  a  similar  one  grown 
on  moist  paper,  and  notice  that  the  root  or  roots  and  the  epicotyl 
grow  from  the  embryo  part  of  the  grain.  Make  a  drawing  of  such 
a  stage.  The  part  of  the  embryo  which  still  remains  embedded  in 
the  endosperm  is  thought  to  be  the  single  cotyledon,  which  remains 
within  the  seed-coat  as  an  organ  to  digest  the  endosperm  and  then 
to  absorb  the  digested  substances  and  transfer  them  by  osmosis  to 
the  growing  parts  of  the  seedling. 

The  first  root  appears  at  the  end  of  the  grain  which  was  attached 
to  the  cob.  Look  for  it  in  grains  before  germination  begins  and 
label  it  hypocotyl  in  drawings.  Recall  that  the  hypocotyl  in  bean 
and  squash  seeds  consisted  of  two  parts  or  regions,  stem  and  root 
(consult  your  drawings).  Could  the  corn  grain  remain  in  the  ground 
if  part  of  the  hypocotyl  grew  into  an  elongated  stem  (that  is,  a  stem 
between  root  and  cotyledons)  as  it  does  in  bean  and  squash  ?  Write 
a  brief  paragraph  comparing  the  corn  grain  and  pea  seed  with 
regard  to  behavior  of  epicotyl,  cotyledons,  and  hypocotyl. 

Make  transverse  and  longitudinal  cuts  through  water-soaked 
(or  preserved  "green")  grains  of  corn.  Identify  endosperm, 
hypocotyl,  epicotyl,  and  cotyledon  by  comparing  with  germination 
stages  studied  as  above.  Make  sketches. 

140.  Other  Seeds.  —  The  following  are  suggested  for  study  by 
pupils  who  satisfactorily  complete  the  preceding  lessons  in  advance 
of  the  class.  Use  dry  and  water-soaked  seeds  and  seedlings  in 
various  stages  of  germination  as  in  preceding  lessons.  Grow  some 
in  flower-pots.  Look  for  the  same  parts  of  embryos  and  compare 
their  development  as  the  seeds  germinate.  Make  drawings  and 
compare  similar  seeds. 

Sunflower  Seed.  —  The  so-called  "flower"  of  the  sunflower  plant 
is  really  a  group  or  head  of  small  flowers,  and  each  flower  forms  one 
"seed."  The  hard  seed-case  is  wall  of  ovary,  corresponding  to  pod 
of  bean.  The  inside  delicate  coat  is  the  real  seed-coat  correspond- 
ing to  that  of  a  bean.  Find  two  cotyledons,  hypocotyl,  and  epi- 
cotyl. Compare  germination  with  that  of  squash  seed. 

Four-o'clock  Seed.  —  One  flower  produces  one  seed  and  the  outer 
coat  is  wall  of  ovary.  The  two  large  cotyledons  are  folded  around 
a  central  mass  of  white  endosperm.  Compare  with  position  of 
endosperm  in  castor-oil  seed. 


152  APPLIED    BIOLOGY 

Buckwheat  Seed.  —  Compare  with  the  four-o'clock. 

Morning  Glory.  —  Embryo  with  two  cotyledons  is  embedded  in 
the  endosperm.  Epicotyl  not  apparent  until  germination. 

Seeds  of  Oak  (acorn),  Horse-Chestnut,  Windsor-  or  Horse-Bean, 
and  Lupines.  —  Compare  with  the  pea  seed,  especially  behavior  in 
germination. 

Peanut.  —  A  nut-like  pod,  comparable  to  pod  of  the  closely  re- 
lated bean. 

Onion  Seed.  —  Embryo  is  embedded  in  endosperm.  Peculiar 
in  that  the  one  cotyledon  lengthens  in  germination  and  raises  the 
seed-coat  with  endosperm  out  of  the  ground.  When  endosperm 
is  all  absorbed  cotyledon  withers  at  top  and  epicotyl  appears 
near  the  surface  of  soil.  Compare  with  germination  of  the  corn 
grain. 

Wheat  and  Oats.  —  Germinate  on  moist  paper  and  in  soil,  and 
compare  with  corn. 

141.  Number  of  Cotyledons  in  Seeds.  —  Some  of  the  seeds 
studied  have  one  and  some  two  cotyledons.  This  difference 
in  number  has  been  made  the  basis  of  classification  of  a  large 
number  of  the  flowering  or  seed-plants  into  the  monocoty- 
ledons (one  cotyledon)  and  dicotyledons  (two  cotyledons). 
In  addition  to  the  number  of  cotyledons,  plants  of  these 
two  groups  are  marked  by  certain  characteristics  of  stems 
and  leaves  and  flowers  which  will  be  described  later  (§161  and 
§197).  Among  the  monocotyledons  are  all  true  grasses  — 
such  as  timothy,  June-grass,  lawn-grasses  —  and  all  the  lilies, 
tulips,  daffodils,  iris,  orchids,  banana-plants,  trillium,  pine- 
apple, palms,  etc.  Among  the  dicotyledons  are  the  familiar 
trees  of  our  deciduous  forests,  the  fruit-bearing  trees  and 
shrubs  of  our  orchards  and  gardens,  all  our  most  common 
garden  vegetables  (except  onions  and  asparagus),  and  most  of 
the  common  wild  flowers  which  do  not  resemble  lilies  in  leaves 
and  flowers. 

Many  of  the  common  cone-bearing  evergreen  trees  (pine, 
yew,  fir,  cypress,  hemlock,  etc.)  have  seeds  with  more  than 
two  cotyledons  (from  3  to  15).  The  cone-like  flowers  (as 
well  as  other  parts)  of  such  plants  are  different  from  the 


STUDJES    OF    SEED-PLANTS  153 

flowers  of  the  ordinary  flowering  plants,  and  they  will  be 
described  later  (§  211). 

142.  The  Work  of  Cotyledons.  —  It  is  evident  from  the 
seedlings  studied  in  the  laboratory  that  cotyledons  have 
several  kinds  of  work.  (1)  They  may  become  leaf-like 
and  for  a  time  serve  as  leaves  (squash,  sunflower).  (2)  They 
may  be  simply  store-houses  of  food  materials  and  without 
the  functions  of  ordinary  leaves,  not  even  getting  out  of  the 
soil  (pea,  acorn).  (3)  They  may  be  of  use  only  as  an  organ 
for  digesting  and  absorbing  food  stored  in  endosperm  around 
the  cotyledons  (corn).  (4)  Some  cotyledons  combine  func- 
tions 1  and  2.  As  an  example,  the  bean  cotyledons  become 
green  and  do  some  work  as  leaves,  and  also  are  stored  with 
food.  This  is  true  in  all  seedlings  which  have  very  thick 
green  cotyledons  rising  above  the  ground  and  no  endosperm. 
(5)  Functions  1  and  3  may  be  combined.  For  example,  the 
cotyledons  of  the  castor-oil  seed  and  onion  are  leaf-like, 
but  they  remain  in  contact  with  the  endosperm  until  it 
has  been  absorbed.  This  is  the  case  in  all  seedlings  with 
cotyledons  which  rise  above  the  ground,  with  endosperm 
in  the  seed,  and  with  the  seed-coats  pushed  out  of  the  soil 
and  remaining  around  the  endosperm  until  it  is  absorbed. 

Observe,  when  opportunity  offers,  seedlings  of  various  wild  and 
cultivated  plants.  It  will  be  interesting  to  reserve  a  page  in  your 
note-book  for  "Work  of  Cotyledons"  and  make  records  in  five 
columns  headed  as  follows:  (1)  As  Leaves ;  (2)  For  Food-storage; 
(3)  As  Absorbers ;  (4)  As  Leaves  and  storage ;  (5)  As  Leaves  and 
absorbers.  (The  numbers  refer  to  those  used  above.) 

Cotyledons  are  usually  equal  in  size,  as  in  bean  and  squash 
seedlings;  but  in  some  species  of  plants  one  is  larger  than 
the  other.  In  some  cacti  both  cotyledons  are  exceedingly 
small  (rudimentary) ;  and  in  still  other  seeds  only  one  coty- 
ledon is  rudimentary. 

143.  The  Epicotyl.  —  The  epicotyl  (sometimes  called 
plumule)  is  found  well  developed  in  some  seeds  before  germina- 


154  APPLIED    BIOLOGY 

tion,  especially  in  seeds  like  bean  with  so  much  stored  food 
that  the  cotyledons  cannot  do  well  the  work  of  leaves  and 
in  seeds  like  corn  and  pea  with  cotyledons  that  do  no  work 
as  leaves.  Under  such  conditions  it  is  important  that  the 
seed  have  a  well-developed  epicotyl  ready  to  put  forth  leaves 
at  an  early  stage  of  germination.  In  seeds  like  the  squash 
with  cotyledons  which  are  leaf -like,  the  leaves  from  the 
epicotyl  are  not  needed  so  early  and  there  is  time  for 
further  development  of  the  small  epicotyl  after  germination 
begins. 

When  you  notice  either  a  large  or  small  epicotyl  in  a  seed,  look 
for  such  relations  of  the  work  of  the  cotyledons  as  is  suggested  in 
paragraph  above. 

In  cases  like  corn  and  pea  where  the  hypocotyl  does  not  push  up 
the  cotyledons,  the  epicotyl  lengthens  and  rises  above  the  ground. 

144.  The  Hypocotyl.  —  It  has  been  noted  in  the  laboratory 
work  that  in  the  seeds  of  squash  and  bean  the  hypocotyl 
of  the  ungerminated  seed  develops  into  part  stem  and  part 
root.     In  these  cases  the  stem  part  of  the  hypocotyl  grows 
in  length  and  pushes  the  cotyledons  upward  out  of  the  soil, 
while  the  root  is  growing  down  into  the  soil.     The  stem  part 
of  the  hypocotyl  of  the  pea  and  corn  does  not  lengthen,  and 
so  the  cotyledons  remain  below  ground,  their  place  as  first 
leaves  above  ground  being  taken  by  the  epicotyl. 

145.  Conditions  of  Germination.  —  It  is  easy  to  demon- 
strate that  water,  oxygen  (from  air),  and  proper  temperature 
are  necessary  for  germination. 

Water.  —  That  water  is  necessary  requires  no  special 
experimental  proof,  for  every  one  knows  that  seeds  kept  dry 
remain  dormant,  and  that  seedlings  do  not  "  come  up  " 
in  gardens  when  the  soil  is  dry,  as  in  midsummer.  Many 
hard-coated  seeds  have  special  ways  of  letting  in  the  necessary 
water,  such  as  the  spongy  mass  at  end  of  castor-bean,  and 
the  holes  in  a  cocoanut.  Other  seeds  must  remain  in  the  soil 


STUDIES    OF    SEED-PLANTS  155 

for  months  until  freezing  or  decay  of  seed-coats  cause  cracks 
into  which  water  can  enter.  Gardeners  often  crack  certain 
hard  seeds,  like  peach  and  various  nuts,  or  file  or  cut  notches 
in  others,  and  still  other  kinds  are  soaked  and  softened  in 
hot  water  before  planting.  By  soaking  seeds  for  various 
lengths  of  time  in  red  ink  it  is  possible  to  trace  the  paths 
taken  by  entering  water,  usually  the  same  as  that  taken  by  the 
sap  while  the  seed  was  developing. 

Proper  Temperature.  —  This  also  requires  no  special  demon- 
stration. Any  one  who  has  ever  been  interested  in  a  garden 
knows  that  warm  weather  is  necessary,  and  that  some  kinds 
of  seeds  will  germinate  in  early  spring  when  the  soil  is  still 
cold  and  that  others  require  warm  soil.  By  simply  placing 
some  seeds  on  a  moist  paper  in  an  ice-box  and  others  of  the 
same  kind  in  warmer  places  one  could  show  the  effect  of 
temperature,  and  after  many  trials  it  would  be  found  that 
there  is  a  best  or  optimum  temperature  for  each  kind  of  seeds. 
Seed-catalogues  and  books  on  gardening  usually  give  the 
necessary  information  regarding  best  temperatures  for  germi- 
nating common  vegetable  and  "  flower  "  seeds. 

Air,  Oxygen.  —  That  the  oxygen  of  the  air  is  necessary 
might  be  expected  because  a  germinating  seed  is  a  living 
plant,  and  we  have  learned  that  all  protoplasm  must  have 
oxygen.  It  can  be  proved  by  placing  seeds  in  a  bottle  and 
pumping  out  the  air  with  an  air-pump,  or  by  filling  the  bottle 
with  pure  nitrogen  gas  made  with  a  chemist's  generator. 
That  carbon  dioxide  is  formed  in  germinating  seeds  can  be 
proved  by  the  lime-water  test  —  simply  stand  a  small  cup 
or  vial  containing  lime-water  in  a  larger,  carefully  stoppered 
bottle  containing  germinating  seeds.  Or  use  the  method 
described  in  §  26,  suspending  seeds  in  a  bag  of  netting  during 
germination,  and  then  after  lifting  out  the  bag,  pour  lime- 
water  into  bottom  of  the  jar. 

The  student  is  advised  to  consult  Chapters  I  and  II  of  Oster- 
hout's  "Experiments  with  Plants"  for  numerous  experiments  and 


156  APPLIED    BIOLOGY 

information  regarding  many  points  concerning  the  physiology  or 
activities  of  germinating  seeds. 

ROOTS  OF  SEED-PLANTS 

146.  Structure  and  Kinds  of  Roots.  —  The  structure  of 
all  roots  of  seed-plants  is  essentially  like  that  described  for  the 
bean  plant;  namely,  bark  or  rind  with  its  outermost  layer  of 
epidermis,  a  central  woody  axis  with  tubes  for  conducting 
water  upward,  and  the  epidermal  root-hairs  near  the  ends  of 
the  young  rootlets.     See  §  67. 

As  to  the  kinds  of  roots,  some  plants  have  a  main  root 
which  grows  downward  and  gives  off  smaller  roots  laterally. 
This  is  a  tap-root;  carrot  and  radish  are  good  examples. 
In  many  plants  no  such  main  root  can  be  distinguished. 
Examples  are  the  much-branched  roots  of  Indian  corn  and 
many  grasses;  and  such  roots  are  called  fibrous.  Tap- 
roots when  thickened  by  food-storage  are  said  to  be  fleshy 
(e.g.,  carrot).  It  should  be  noted  that  a  carrot  or  radish 
as  seen  in  markets  is  not  all  root,  for  the  upper  end  is  thick- 
ened stem  with  a  bud  for  next  season's  growth. 

(L)  Pull  up  as  many  different  kinds  of  cultivated  plants  and 
weeds  as  possible,  examine  the  roots,  and  make  sketches  of  the 
different  forms  of  roots  seen. 

147.  Roots   as   Anchors   and    as   Absorbers.  —  The    two 
primary  functions  of  roots,  anchoring  plants  in  soil  and  ab- 
sorbing water  with  useful  substances  in  solution,  are  carried 
on  in  all  roots  essentially  as  described  in  the  preceding  chapter 
on  the  work  of  the  bean  plant  (see  §  86). 

148.  Roots  Fitted  for  Growth  in  the  Soil.  —  The  root-cap, 
already  studied  (§  68),  protects  the  growing  tip  from  injury 
as  the  root  pushes  between  the  particles  of  the  soil.     The 
fact  that  increase  in  length  takes  place  near  the  growing 
tip  (Fig.  34)  may  be  of  advantage  in  curving  to  pass  around 
stones    and    other    obstructions,    and    in    turning    towards 
moisture  and  food.      The  primary  root   grows    downward. 


STUDIES    OF   SEED-PLANTS  157 

Its  branches  (secondary  roots)  grow  out  sidewise,  and  the 
branches  grow  in  various  directions,  so  that  they  finally  result 
in  a  complicated  root-system  with  distribution  of  roots 
through  a  large  mass  of  soil.  This  better  fits  roots  for 
absorbing  from  the  soil  and  for  anchoring  the  plant. 

149.  Extent  of  Roots.  —  If  one  pulls  up  a  plant,  such  as 
most  common  weeds,  it  becomes  evident  that  if  all  the  little 
roots  were  cut  and  placed  end  to  end,  the  total  extent  would 
be  very  great.  But  this  would  not  give  an  adequate  idea 
of  the  total  extent,  for  hundreds  of  small  roots  are  broken 
in  pulling  up  a  plant,  as  can  be  proved  by  comparing  a 
plant  pulled  from  the  soil  of  a  pot  with  one  which  has  been 
grown  without  soil.  It  has  been  estimated  that  the  total 
length  of  roots  of  a  corn  plant  is  over  a  thousand  feet  and 
that  a  large  squash  plant  has  many  miles  of  roots.  In 
ordinary  garden  soil  roots  are  often  found  several  feet  below 
the  surface.  In  dry  regions  roots  of  alfalfa  (a  kind  of  clover) 
are  said  to  go  down  twenty  to  thirty  feet.  However,  the  great 
majority  of  roots  of  most  plants  are  not  far  below  the  surface 
of  the  soil,  but  they  extend  long  distances  horizontally. 

In  addition  to  the  great  length  of  roots,  the  root-hairs 
may  increase  the  absorbing  surface  so  much  that  a  plant  with 
roots  of  a  total  length  of  one  foot  would  equal  roots  of  from 
twenty  to  fifty  feet  without  root-hairs.  These  are  estimates 
made  by  botanists  who  have  carefully  examined  roots  of 
various  plants. 

The  tendency  of  many  roots  to  extend  far  from  the  stem 
makes  transplanting  more  difficult  because  so  many  small 
roots  are  necessarily  cut  off  and  left  in  the  soil.  Gardeners 
and  nurserymen  overcome  this  by  transplanting  young  plants 
several  times,  thus  forcing  the  growth  of  small  roots  in  a 
cluster  near  the  stem.  In  order  to  prepare  large  trees  for 
moving,  it  is  often  necessary  to  begin  several  years  before 
and  cut  off  annually  a  few  large  roots  several  feet  from  the 
stem  so  that  new  branch  roots  will  be  grown  near  the  stem 


158  APPLIED    BIOLOGY 

before  the  last  long  roots  are  cut  at  the  time  of  removal  to 
a  new  location.  One  firm  of  tree  dealers  keeps  on  its  grounds 
large  trees  worth  several  hundred  dollars  each,  which  for 
many  years  have  had  their  roots  cut  back  in  preparation  for 
sale  and  removal. 

150.  Food  Storage  in  Roots.  —  It  is  a  well-known  fact 
that,  in  addition  to  the  two  primary  functions  of  anchoring 
and  absorbing,  many  roots   (turnip,  beet,   carrot,  parsnip, 
sweet  potato,  etc.)  become  thickened  and  thereby  useful  as 
food  for  man  and  some  animals.     However,   the  primary 
purpose  is  not  storage  for  man  and  animals,  but  for  the  plant's 
use  later.     Many  plants  which  thus  store  food  in  roots  use 
it  in  the  second  year  for  the  development  of  flowers  and  seeds, 
and  then  the  plants  die.     Such  are  biennials  or    two-year 
plants.     Other  plants  which  live  many  years  (e.g.,  rhubarb 
and  asparagus)  store  food  every  year  during  the  summer  to 
be  used  for  early  growth  in  the  next  spring.     In  fact,  when 
one  sees  either  cultivated  or  wild  plants  growing  rapidly 
in  very  early  spring,  it  is  very  probable  that  food  stored 
in  the  roots  furnishes  the  materials  for  such  rapid  growth. 
This  is  the  explanation  of  the  early  flowering  of  crocuses, 
daffodils,  tulips,  hyacinths,  snowdrops,  spring  beauty,  and 
numerous  other  plants  which  bloom  in  very  early  spring. 

Roots  of  many  species  of  plants  have  long  been  one  source 
of  human  food ;  and  in  recent  years  the  European  practice 
of  feeding  roots  of  turnips,  mangels  (a  kind  of  beet),  ruta 
bagas  (a  kind  of  turnip),  and  carrots  as  a  part  of  the  food 
for  farm  animals  has  been  adopted  in  many  places  in  America. 

151.  Adventitious  roots  are  those  given  off  by  the  stem 
or  parts  of  the  plant  other  than  roots.     The  aerial  roots  of 
poison  ivy,  the  brace-roots  of  corn,  and  parasitic  roots  of 
dodder    are  good  examples.     Cuttings  of  stems  of  willow, 
poplar,  and  garden  tradescantia  placed  in  bottles  with  water 
will  soon  form  adventitious  roots.     Gardeners  start  many 
plants  like  geranium,  coleus,  begonia,  and  numerous  other 


STUDIES  OF  SEED-PLANTS  159 

greenhouse  plants  from  slips  or  cuttings  of  stems  which 
form  such  roots  when  set  in  moist  sand.  In  order  to  prevent 
erosion  along  river  banks,  stakes  cut  from  branches  of  willows 
are  often  driven  into  the  soil  and  soon  form  adventitious 
roots  and  put  forth  leaves,  forming  young  trees.  Straw- 
berry runners  (which  are  branches  of  stems  trailing  along 
the  ground) ,  blackcap  raspberry  stems,  and  numerous  common 
weeds  "  take  root";  that  is,  form  adventitious  roots  at  the 
nodes.  Bryophyllum  leaves  and  those  of  some  begonias 
form  roots  when  placed  on  moist  soil,  and  develop  new 
plants. 

152.  Root-tubercles.  —  These  were  mentioned  in  connec- 
tion with  the  bean  root  (§  67).     Only  by  the  aid  of  the  bac- 
teria which  live  in  the  root-tubercles  is  it  possible  for  plants 
to  make  use  of    the  nitrogen  which  constitutes  over  two 
thirds  of  the  air.     These  bacteria  have  the  peculiar  power 
of  combining  nitrogen  with  other  elements  derived  from  the 
soil,  and  the  nitrogen  compounds  thus  formed  are  absorbed 
by  the  roots  and  used  by  the  plant  just  as  it  uses  the  nitrogen 
in  manures  and  in  such  soil  fertilizers  as  nitrate  of  soda. 

153.  Special  Adaptations  of  Roots.  —  The  great  majority 
of  plants  have  roots  whose  functions  are  (1)  anchoring  the 
plant,  and    (2)    absorbing  water   and   dissolved   substances 
from  the  soil ;    but   some  roots  have  interesting  modifica- 
tions of  structure  to  adapt  them  to  other  kinds  of  work. 
Such  modified  structures  which  are  fitted  to  special  work  or 
functions   are   called   adaptations.     Some   of  the  most  im- 
portant of  such  specialized  root  structures  are  aerial  roots, 
prop-roots,  water  roots,  breathing  roots,  and  parasitic  roots. 
These  will  be  described  in  this  order. 

(1)  Aerial  Roots.  —  Many  plants  develop  roots  in  the  air; 
that  is,  above  the  soil.  Such  roots  may  serve  as  clinging 
organs,  as  in  poison  ivy,  trumpet-creeper,  and  English  ivy ; 
and  on  some  plants  they  are  important  absorbers  of  water. 
Many  so-called  "  air-plants,"  from  tropical  countries,  such 


160  APPLIED  BIOLOGY 

as  some  orchids  and  ferns  common  in  greenhouses,  are  at- 
tached to  branches  of  trees,  but  do  not  have  parasitic  roots 
penetrating  and  absorbing  sap  from  the  supporting  tree. 
Instead,  their  roots  absorb  water  from  the  moist  air  and  also 
that  which  drips  from  leaves  and  branches. 

(2)  Prop-Roots.  —  On  some  plants  roots  start  from  the 
stem  above  ground  and  grow  down  into  the  soil  so  as  to 
form  bracing  or  supporting  structures.     Every  one  who  has 
cultivated  Indian  corn  must  have  noticed  the  brace-roots 
or  prop-roots  which  start  from  nodes  above  the  soil  and  grow 
down  into  the  soil.     These  help  the  ordinary  roots  in  bracing 
the  tall  stalk  (stem)  against  winds,  and  also  it  is  probable 
that  they  advantageously  increase  the  number  of  rootlets 
available  for  absorption  of  water  while  the  plant  is  maturing. 
Other  examples  of  roots  forming  supports  or  braces  are  in 
the  famous  banyan  trees  of  India,  whose  aerial  roots  descend 
from  branches  to  the  ground  and  form  many  additional  sup- 
ports and  absorbing  roots  for  the  tree.     A  single  tree  may 
cover  a  space  300  feet  in  diameter.     Mangrove  trees,  which 
grow  along  the  sea-coast  in  tropical  regions,  form  brace- 
roots  from  both  main  stem  and  branches,  and  these  roots 
grow  down  into  the  mud. 

(3)  Water  Roots.  —  Roots  of  many  plants  cannot  live  in 
water.     This  is  the  reason  why  many  trees  die  when  a  new 
dam  has  raised  the  water  level.     Some  plants,  however,  have 
roots  which  can  live   in  either  soil  or  water,  e.g.,  willows. 
Many  aquatic  plants  have  roots  adapted  to  living  in  water 
only,  e.g.,  duckweed  and  water-hyacinth. 

Many  aquatic  roots  have  no  root-hairs.  Such  roots  are 
usually  much  branched  and  have  the  required  amount  of  ab- 
sorptive surface  without  root-hairs.  Some  aquatic  plants 
can  live  entirely  without  roots,  absorbing  by  their  sub- 
merged stems  and  leaves. 

(4)  Breathing  Roots.  —  Reference  has  already  been  made 
(§  105)  to  the  fact  that  roots  get  some  oxygen  from  air  in 


STUDIES  OF  SEED-PLANTS  161 

the  soil ;  but  the  mangrove  trees  mentioned  above  often  grow 
in  mud  covered  with  water,  which  interferes  with  aeration. 
Certain  species  of  mangroves  overcome  this  difficulty  by 
the  development  of  branch  roots  which  grow  up  into  the 
air  out  of  the  mud  and  water.  These  roots  have  special 
air-passages  for  conducting  air  down  into  the  roots,  and 
they  take  up  oxygen  and  discharge  carbon  dioxide  as  do 
other  plant  tissues  exposed  to  the  air;  that  is,  they  breathe. 

(5)  Parasitic  Roots.  —  Many  species  of  plants  have 
peculiar  roots  adapted  for  penetrating  the  epidermis  of  other 
plants  and  thus  forming  a  close  attachment  for  absorbing 
the  necessary  foods.  Examples  are  the  mistletoe,  which 
grows  on  the  bark  of  oaks,  apple,  and  other  trees ;  and  the 
common  dodder,  which  has  root-like  branches  penetrating 
other  plants  from  which  it  can  absorb  suitable  foods.  These 
are  cases  of  plant  parasites;  and  this  condition  in  which 
one  plant  (the  parasite)  absorbs  its  food  from  another  plant 
(the  host)  is  parasitism.  The  mistletoe  has  pale  green  leaves 
and  can  make  some  starch,  so  it  is  only  in  part  a  parasite. 
The  dodder  has  no  roots  in  the  soil,  only  some  very  small 
scale-like  rudiments  of  leaves  without  chlorophyll,  and  a  yel- 
low, string-like  stem  with  very  little  chlorophyll.  Evidently 
it  must  depend  upon  other  plants  for  the  foods  which  ordinary 
plants  get  from  the  soil  and  make  in  their  green  leaves.  In 
recent  years  the  dodder  has  been  accidentally  planted  in 
many  fields  by  seed  mixed  with  those  of  clover  and  alfalfa, 
and  has  caused  great  damage  by  absorbing  food  from  the 
plants  with  which  it  makes  a  root-like  connection.  For 
more  facts  concerning  the  relation  of  this  weed  to  agriculture, 
read  Farmers'  Bulletin  306,  "  Dodder  and  its  Relation  to 
Farm  Seeds." 

154.  Binding  Action  of  Roots.  —  After  any  heavy  rain- 
storm one  can  see  that  bare  soil,  such  as  on  roads  and  culti- 
vated fields,  has  been  washed  away  where  water  has  flowed 
rapidly,  while  grassy  sod  has  not  been  so  affected.  This 


162  APPLIED  BIOLOGY 

illustrates  an  important  work  of  roots,  that  of  holding 
particles  of  soil  together.  Cutting  a  forest  from  a  hillside 
or  sloping  land  allows  rapid  washing,  and  large  tracts  of 
valuable  land  have  been  ruined  by  removal  of  fertile  soil  and 
by  formation  of  deep  gullies.  See  books  and  pamphlets 
on  forestry. 

The  same  erosion  often  occurs  when  cultivated  fields  are 
bare  during  the  winter.  Even  if  deep  furrows  or  gullies 
are  not  formed,  it  is  evident  that  the  muddy  water  which 
always  flows  down  bare  slopes  is  carrying  away  large  quanti- 
ties of  the  fertile  top  soil.  This  has  not  been  sufficiently 
understood  by  many  farmers  in  the  past,  but  its  importance 
has  been  so  clearly  demonstrated  that  the  scientific  farmer 
of  to-day  avoids  leaving  bare  during  the  winter  cultivated 
soil  which  is  sloped  so  that  water  tends  to  wash  it  away. 
Protection  against  erosion  is  accomplished  by  planting  in 
the  late  summer  some  winter  crop,  either  after  harvesting 
a  cultivated  crop  like  corn  and  vegetables,  or  between  the 
rows  after  the  last  cultivation,  or  in  cultivated  orchards 
and  vineyards.  This  winter  crop  may  be  wheat,  rye,  or 
grasses  to  be  harvested  in  the  following  summer,  or  a  "  cover 
crop  "  of  certain  clovers,  vetches,  rye,  and  other  plants  which 
make  great  root  growth  in  the  autumn,  largely  prevent 
erosion  by  the  rains  of  the  winter  and  early  spring,  and  add 
to  the  fertility  of  the  soil  if  plowed  under  in  the  next  year's 
cultivation. 

155.  Propagation  from  Roots.  —  While  many  stems  readily 
develop  roots,  most  roots  do  not  form  stems.  Exceptions 
are  sweet  potato,  which  is  a  thickened  secondary  root,  and 
roots  of  the  osage-orange  tree.  In  order  to  start  new  plants 
of  sweet  potato  or  osage-orange,  it  is  simply  necessary  to 
bury  root-cuttings  in  favorable  soil.  A  sweet  potato  will 
form  stems  on  any  part  of  its  surface ;  but  roots  like  carrot, 
beet,  horse-radish,  and  turnip  soon  die  when  the  stem  end 
or  bud  is  removed.  Some  plants  (e.g.,  dahlia)  form  a  cluster 


STUDIES   OF  SEED-PLANTS  163 

of  roots  at  the  base  of  the  stem,-  but  each  one  of  these  can 
grow  a  new  stem  only  at  the  upper  or  stem  end.  Many  of 
the  root-like  structures  from  which  new  stems  originate 
are  really  underground  stems  (see  §  176).  A  familiar  ex- 
ample is  the  ordinary  potato. 

STEMS  OF  SEED-PLANTS 

We  have  seen  in  the  case  of  the  bean  plant  that  the 
stem  has  certain  definite  functions  in  the  life  of  the  plant. 
Most  of  the  statements  made  concerning  the  structure 
and  functions  of  the  stem  of  the  bean  plant  (§  69)  are 
true  of  higher  plants  in  general.  However,  stems  of  many 
plants  have  modified  structure  and  special  functions;  and 
we  should  have  a  very  limited  knowledge  and  appreciation 
of  plant  stems  if  we  confined  our  study  to  one  kind  of  stem. 
Hence,  the  following  lessons  provide  for  study  of  the  more 
important  facts  concerning  stems  which  have  not  already 
been  presented  in  connection  with  the  bean  plant. 

156.  Twigs.  —  This  is  a  popular  term  applied  to  pieces 
of  stems,  or  to  small  branches  of  trees  and  shrubs.  They  are 
very  convenient  for  study,  because  they  illustrate  on  a 
small  scale  the  general  structure  of  the  entire  stem  of  the 
plant  from  which  they  are  taken. 

Horse-chestnut  Twig.  —  (L)  Or  twigs  of  hickory,  ash,  box-elder, 
ailanthus,  linden,  sycamore,  etc.  Note  that  the  bud  at  the  end  of 
the  twig  (terminal  bud)  is  larger  than  the  buds  along  the  sides 
(lateral  buds).  Observe  the  positions  of  the  buds  along  the  stem  — 
are  they  alternate  or  opposite  to  each  other?  What  is  the  relation 
of  the  arrangement  of  each  successive  pair  of  buds  to  each  other  ? 
Observe  the  large  horseshoe-shaped  scars  immediately  beneath  the 
buds.  These  are  leaf-scars.  What  is  the  meaning  of  the  markings 
on  the  scars  ?  What  is  the  position  of  a  bud  with  reference  to  the 
leaf  ?  Look  for  other  kinds  of  scars  on  the  twig.  The  very  small 
scars  scattered  irregularly  over  the  stem  are  lenticels  or  breathing 
pores  (§  105).  The  nature  of  the  rings  of  scars  will  be  made  evident 
after  a  study  of  the  bud.  Often  one  can  find  a  large  scar  in  the  fork 


164 


APPLIED  BIOLOGY 


of  a  branch ;  this  is  a  flower-scar.  On  a  much  larger  twig,  observe 
the  arrangement  of  the  branches  —  how  does  it  compare  with  the 
arrangement  of  the  buds  already  noted?  This  is  what  we  should 
expect  if  the  branches  are  merely  developed 
buds,  as  stated  in  the  definition  of  a  bud  in  the 
next  section. 

Make  a  sketch  of  the  twig  which  has  been 
studied,  showing  as  many  of  above  points  of 
structure  as  possible.  Compare  with  as  many 
twigs  as  possible  —  lilac,  apple,  cherry,  tulip- 
tree,  butternut,  beech,  and  others  named  above 
as  possible  substitutes  for  the  horse-chestnut 
branches. 

157.  Buds.  —  A  bud  is  a  growing  point 
on  a  plant  which  when  conditions  are  favor- 
able will  develop  into  a  leafy  branch,  a 
flower,  or  both.  In  temperate  regions  where 
plants  must  withstand  a  long  cold  winter, 
trees  and  shrubs  have  formed  the  habit 
of  covering  and  protecting  their  growing 
points  with  scales,  hair,  or  gummy  sub- 
stances. The  horse-chestnut  bud  is  a  good  example  of  a  scaly 
or  winter  bud,  and  similar  ones  can  be  seen  on  any  of  our  hardy 
shrubs  or  trees.  In  annuals  and  herbaceous  perennials  of  tem- 
perate climates  and  in  tropical  countries  where  the  growing 
season  is  not  interrupted  by  a  winter  season,  with  its  sudden 
changes  of  temperature  and  moisture,  the  buds  as  a  rule  have 
either  very  little  covering  of  scales  or  hairs,  or  none  at  all 
(naked  buds). 

Buds  of  Horse-chestnut.  —  (L)  Or  hickory,  lilac,  elm,  or  any  large 
bud.  Examine  horse-chestnut  buds  and  note  that  they  are  all 
covered  with  sticky  brown  scales  which  overlap  as  do  the  shingles 
on  a  house.  Examine  a  bud  cut  longitudinally.  Suggest  a  use  for 
the  sticky  substance.  Carefully  remove  the  scales  from  a  bud  and 
observe  the  ring  of  scars  left  on  the  stem.  This  marks  the  end  of 
the  year's  growth  of  the  stem.  Look  along  the  stem  for  other 
similar  rings  of  scars  —  how  many  do  you  find  ?  What  relation  do 
the  number  of  such  rings  of  scars  bear  to  the  age  of  the  twig  ? 


FIG.  43.  Left  twig, 
horse-chestnut ; 
right  one,  hickory. 
(From  Gray.) 


STUDIES  OF  SEED-PLANTS  165 

Note  that  the  bud-scales  are  arranged  in  pairs,  and  that  they  be- 
come softer  and  more  leaf -like  as  you  approach  the  center.  When 
all  the  scales  have  been  removed,  there  remains  a  soft  woolly  object, 
the  young  stem  and  leaves.  Suggest  a  use  for  the  hairy  substance. 
Sometimes  the  terminal  bud  is  a  flower-bud.  Compare  what  you 
have  found  in  a  closed  bud  with  what  you  find  in  an  opening  bud 
on  a  twig  which  has  been  standing  in  water  in  a  warm  room.  Ob- 
serve that  such  a  bud  has  swelled,  some  of  the  outer  scales  have  fallen 
off,  leaving  a  fresh  ring  of  scars  on  the  stem,  the  inner  scales  have 
become  larger  and  more  leaf -like,  the  shoot  has  elongated,  the  leaflets 
are  separating  and  unfolding. 

Compare  buds  from  various  trees  and  shrubs. 

Leaves  are  formed  folded  in  buds.  This  is  necessary  for 
two  reasons:  (1)  leaves  in  a  bud  must  take  up  as  little 
space  as  possible;  (2)  young  tender  leaves  must  have  as 
little  exposed  surface  as  possible  to  the  dry  and  cold  air  of 
winter.  The  folding  of  the  leaves  in  the  bud  is  known  in 
botany  as  vernation. 

Examine  buds  of  various  trees  and  shrubs  such  as  lilac,  tulip-tree, 
hickory,  beech,  currant,  etc.,  for  types  of  vernation.  Also  examine 
naked  buds. 

Bud-like  Structures.  —  The  cabbage  head  and  the  onion 
bulb  both  illustrate  the  structure  of  a  bud;  but  it  should 
be  noted  that  they  are  not  true  buds,  for  they  represent 
fully  developed  shoots  with  very  short  stems.  Examine 
both  of  them  in  longitudinal  sections  obtained  by  cutting 
through  the  centers. 

158.  Position  of  Buds  and  Branches.  —  In  the  study  of 
the  horse-chestnut  twig,  attention  was  called  to  the  fact 
that  buds  appear  in  the  axils  of  leaves  and  are  either  terminal 
or  lateral.  But  buds  often  occur  on  other  parts  of  plants, 
for  instance,  on  the  root  or  along  the  trunks  of  trees  and  even 
on  the  notches  along  the  margin  of  leaves  (e.g.,  in  Bryophyl- 
lum).  Such  buds  are  abnormal  in  position  and  are  known 
as  adventitious.  They  develop  on  some  trees  when  large 
branches  have  been  cut  off,  and  advantage  is  taken  of  this 


166  APPLIED  BIOLOGY 

fact  by  pollarding  willow  trees  and  thus  causing  them  to 
produce  from  adventitious  buds  the  straight  and  slender 
branches  suitable  for  making  baskets.  Some  trees  are 
pollarded  in  order  to  get  ornamental  trees  with  rounded 
bushy  tops.  There  are  many  trees  and  shrubs  which  have 
more  than  one  bud  in  the  axil  of  each  leaf ;  but  only  one  of 
these  is  axillary,  while  the  others  are  supernumerary  or 
accessory.  This  is  well  shown  in  the  box-elder  and  butternut. 
On  currant  bushes  there  are  usually  three  buds  at  the  end  of 
each  twig,  the  central  one  being  the  terminal  bud  and  the 
other  two  accessory. 

Opposite  Branching.  —  When  the  buds  are  in  pairs  at  each 
node  and  opposite  each  other,  there  is  a  four-rowed  arrange- 
ment on  the  twig ;  but  owing  to  the  failure  of  some  of  the 
buds  to  develop  and  the  death  of  some  young  twigs,  this 
arrangement  is  seldom  perfect  on  fully  grown  branches. 
As  examples,  observe  maple,  ash,  horse-chestnut,  and  lilac. 

Alternate  Branching.  —  When  the  buds  are  arranged  one  to 
each  node,  there  may  be  two,  three,  five,  or  more  rows.  In 
passing  from  any  bud  to  the  next  above  on  the  stem  we  go 
spirally  around  the  stem.  Examples  are  hickory,  elm,  beech, 
ailanthus,  and  walnut.  Alternate  is  more  common  than 
opposite  branching. 

159.  Tree  Forms.  —  If  the  terminal  bud  takes  the  lead 
in  growth,  the  result  is  a  tnee  like  the  spruce  or  hemlock,  in 
which  the  straight  main  trunk  can  be  traced  to  the  very  top 
of  the  tree,  and  with  relatively  small  lateral  branches.  When 
the  branches  from  the  lateral  buds  develop  rapidly,  the  result 
is  a  round  top  tree,  like  the  apple,  in  which  the  main  trunk 
cannot  be  distinguished  from  main  branches.  Most  of 
our  common  forest  trees  are  intermediate  between  these  two 
types.  Observe  various  trees  and  decide  to  which  of  these 
classes  they  belong.  Also  notice  that  the  characteristic 
contour  of  a  tree  or  shrub  depends  both  upon  its  mode  of 
branching  and  upon  the  angle  which  the  branch  makes 


STUDIES   OF  SEED-PLANTS  167 

with  the  main  stem,  e.g.,  compare  Lombardy  poplars  with 
ordinary  trees  of  our  forests  and  parks. 

160.  Elongation  of  Stems.  —  Careful  studies  have  shown 
that  elongation  is  most  rapid  just  below  the  growing  tip  of 
stems  and  their  branches,  but  that  elongation  occurs  else- 
where to  some  extent  so  long  as  hard  woody  tissue  has  not 
developed   in   the   internodes   of  the   stem.     For   example, 
the  distance  between  the  first  true  leaves  and  the  cotyledons 
increases  for  some  time  while  the  tissues  in  this  internode 
are  soft.     If  one  measures  the  distance  between  marked 
branches  on  a  young  fruit-tree  in  early  spring  and  again  in 
autumn,  it  will  be  found  that  the  older  branches  have  not 
grown  farther  apart  or  higher  from  the  ground;    but  the* 
branches   which   appear   during   the   growing   season   grow 
farther  apart  during  some  weeks  while  the  tissues  are  soft. 
This  is  a  matter  of  practical  importance  for  one  who  culti- 
vates trees.     In  pruning  (§  166)  branches  must  be  left  at 
the  height  where  it  is  desired  to  have  them  when  the  tree  is 
fully  grown.     Also  by  cutting  the  ends   (terminal  buds)   of 
the  main  stem  and  of  its  branches  it  is  possible  to  control 
the  height  of  the  tree,  and  thus  keep  fruit-bearing  branches 
near  the  ground  for  convenience  in  collecting  fruit. 

161.  Stem    of    Monocotyledons.  —  The    bean    plant   has 
the  type  of  stem  found  in  all  dicotyledons  and  cone-bearing 
plants.     The  stem  of  monocotyledons  is  different  in  that 
its  fibro-vascular  bundles  (bundles  of  fibers  and  wood-tubes) 
are  arranged  irregularly  in  the  pith  (Fig.  44),  and  not  in 
rings  as  in  stems  of  dicotyledons  (Fig.  48).     Moreover,  the 
monocotyledon  stem  does  not  increase  in  diameter  each  year 
by  addition  of  layers  to  the  outside  of  the  wood,  but  instead 
grows  to  the  full  diameter  during  its  primary  growth.     Thus 
the  trunks  of  palms  and  cocoanut  trees  are  nearly  the  same 
diameter  almost  to  the  leafy  top ;    and  although  the  trees 
continue  to  grow  for  many  years,  there  is  little  increase  in 
diameter  except  near  the  growing  top.     Such  increase  as 


168 


APPLIED  BIOLOGY 


.FiG.  44.  Transverse  section 
of  corn  stem  showing  rind 
(r) ,  pith  (p) ,  and  numerous 
fibro-vascular  bundles  (v). 


does  occur,  as  in  some  palms,  is  due  to  expansion  of  the 
first-formed  cells,  not  to  new  layers  of  cells  added  between  the 
bark  and  wood,  as  in  the  dicotyledons 

(§  162). 

Structure  of  Corn  Stalk.  —  (L)  Cut  a 
dry  corn  stalk  (stem)  transversely.  Note 
the  smooth  rind  (not  true  bark)  on  the  out- 
side, and  inside  the  rind  is  the  soft  pith 
throughout  which  the  fibro-vascular  bun- 
dles are  scattered  irregularly.  Where  are 
the  bundles  largest  ?  Where  are  they  most 
abundant  ?  Suggest  a  reason  for  such  ar- 
rangement. 

Split  a  stalk  lengthwise,  and  the  fibro- 
vascular  bundles  appear  as  long  threads 
which  are  easily  pulled  out.  Trace  these 
long  threads  through  nodes.  Do  they  all 

continue  upward  or  do  some  of  them  turn  toward  the  rind  at  the 

nodes  ?    Try  to  trace  some  of  the 

bundles  out  into   the   bases   of 

leaves  left  on  the  nodes. 

Stand  a  piece  of  green  corn- 
stem  for  a  short  time  in  red  ink, 

and  then  cut  both  transversely 

and    longitudinally     in    several 

places,  and  find  out  where  the 

colored  liquid  ascends  the  stem. 
Examine  with  a  microscope  a 

thin   cross    section   cut   from  a 

green  stem  or  from  one  preserved 

in  alcohol  or  formalin  solution. 

Observe    that   all   parts  of  the 

section  are   composed   of   cells, 

most  of  which  are  thin-walled 

pith-cells.     Scattered  irregularly 

among  these  are  denser  areas  of 

cells,  the  fibro-vascular  bundles.     FlG-   45-      Transverse    section    of    a 

Notice  that  the  bundles  are  very        S^*  —  ±^"£2 

small  and  closely   packed   near        a>  d>  sp>  ducts  or  wood-tubes ;  str, 

the  margin,  thus  making  the  firm         strengthening  fibers  ;  «/,  sieve-tubes  ; 

rind,  while  the  bundles  toward        wp,  woody  pith.     (From  Osterhout.) 


STUDIES   OF  SEED-PLANTS  169 

the  center  are  larger  and  more  scattered.  Make  a  sketch  showing 
the  above  points.  Carefully  examine  a  bundle  near  the  center  of 
the  section  of  the  stem,  and  note  that  it  consists  of  two  parts,  (1)  that 
directed  toward  the  center  of  the  stem  is  made  up  of  wood-cells  sur- 
rounding the  air-  and  water-tubes,  while  (2)  the  outer  part  of  the 
bundle  consists  of  wood-cells  surrounding  tubes  known  as  sieve-tubes. 
Carefully  observe  the  structure  of  the  bundle,  and  compare  with 
Fig.  45. 

162.  Stem  of  Dicotyledons.  —  While  a  growing  stem  is 
elongating  in  a  given  internode,  the  arrangement  of  the  tissues 
remains,  practically  the  same  as  described  for  the  young  bean 
stem  (§  70).  This  is  the  primary  growth,  and  is  the  condition 
in  many  annual  dicotyledonous  plants.  But  in  many  plants 
which  live  for  a  number  of  years  the  stem  increases  in  diameter 
long  after  the  elongation  has  ceased  in  a  given  internode, 
and  this  increase  in  diameter  after  the  first  woody  tissue  has 
been  formed  is  due  chiefly  to  the  formation  of  new  wood- 
cells  between  the  cambium  and  the  first-formed  wood.  At 
the  same  time  that  the  cambium  is  adding  new  wood-cells,  it 
adds  new  cells  to  the  inner  layer  of  the  bark.  These  addi- 
tional layers  of  wood  and  bark  constitute  the  secondary 
growth  of  the  stem.  It  is  the  common  method  of  increasing 
diameter  of  the  hard  stems  of  perennial  dicotyledonous  and 
cone-bearing  plants.  This  growth  of  new  layers  commonly 
occurs  annually  and  results  in  the  rings  of  wood  seen  in 
transverse  sections  of  many  trees.  These  annual  rings  are 
clearly  marked  in  many  trees  by  the  extra-large  wood-tubes 
formed  during  the  rapid  growth  in  the  beginning  of  the 
growing  season. 

The  age  of  a  tree  cannot  always  be  accurately  determined 
by  counting  the  annual  rings,  for  it  has  been  found  that  some- 
times two  rings  are  formed  in  a  single  year.  This  may  happen 
if  growth  is  checked  for  a  time  in  early  summer,  as  by  drought 
or  destruction  of  leaves  by  caterpillars,  and  begins  again 
later  in  the  summer.  In  tropical  regions,  where  conditions 
for  growth  are  uniform  throughout  the  year,  it  may  happen 


170 


APPLIED  BIOLOGY 


in  certain  cone-bearing  trees  that  no  annual  rings  are  marked 
off,  because  at  no  time  is  there  extra-rapid  growth  such  as 
produces  the  large  wood-tubes  in  trees  of  the  temperate  zone 
in  the  spring. 

163.  Gross  Structure  of  Dicotyledon  Stems.  —  (L)  Materials : 
Stems  of  oak,  pine,  beech,  maple,  and  as  many  others  as  obtainable, 

selecting  stems  of  various 
diameters,  from  half  inch 
to  two  inches  or  more. 
These  should  be  cut  off 
with  a  fine-tooth  saw  and 
then  smoothed  by  rubbing 
on  sandpaper  which  has 
been  glued  or  tacked  to  a 
board.  Also,  some  sec- 
tions or  cut-ends  of  twigs, 
one,  two,  and  three  years 
old.  In  many  high  schools 
the  boys  who  know  how 
to  use  carpenters'  tools 
prepare  a  neat  collection 
of  woods.  Dead  and 
well-dried  (' '  seasoned ' ' ) 
branches  are  best. 

The  hand-lens  shows 
that  the  annual  rings  are 
marked  off  by  large  pores 
(the  wood-tubes).  The 
large  wood-tubes  of  the  in- 
ner ring  were  formed  while  the  stem  was  soft  and  still  elongating  (dur- 
ing its  primary  growth) .  Usually  only  this  ring  will  be  found  in  stems 
taken  during  the  first  season's  growth.  Compare  stems  of  varying 
diameters,  and  count  the  rings.  Each  season's  growth  begins 
with  the  large  tubes,  formed  when  growing  rapidly  in  spring,  and 
extends  outward  to  the  next  ring  of  large  wood-tubes.  In  many 
woody  stems  some  pith  remains  in  the  center. 

Most  woody  stems  have  delicate  radiating  lines,  starting  from  the 
pith  at  the  center  ;  and  in  older  stems  some  of  these  radiating  lines 
start  in  annual  rings  and  do  not  extend  to  the  center.  These  lines 
are  the  pith-rays  or  medullary  rays,  which  are  so  named  because  they 
are  composed  of  the  same  kind  of  cells  as  those  found  in  the  central 


FIG.  46.  From  four-year-old  pine  stem  cut  in 
winter.  Annual  rings  (1,  2,  3,  4)  ',  pith-rays 
(m) ;  cambium  (c)  ;  bark  with  bast  (6)  ;  lines 
between  wood  of  successive  years  (i)  ;  center 
of  stem  (p).  (From  Strasburger.) 


STUDIES   OF  SEED-PLANTS 


171 


pith  or  medulla.  In  some  woods,  if  examined  with  the  unaided  eye, 
the  rays  appear  to  extend  only  to  the  bark ;  but  in  other  stems  (as 
beech)  the  rays  extend  out  into  the  bark.  The  fact  is  that  the  same 
kind  of  cells  extend  from  the  pith  out  into  the  bark  in  all  dicotyledon 
stems,  but  only  in  cases  like  beech  are  they  compressed  in  the  bark 
into  plates  visible  to  the  unaided  eye.  v 

Notice  the  difference  in  color  between  the  outer  and  inner  annual 
rings.  The  lighter  colored  outer  part  of  the  wood  is  the  sap-wood, 
so  called  because  it  is  active  in  transporting  water  and  in  freshly 
cut  stems  appears  to  be  filled  with  sap.  Some  of  its  cells  contain 
living  substance.  The  darker  central  wood  is  the  heart-wood, 
composed  entirely  of  dead  cells.  How  do  you  explain  the  fact  that 
many  large  trees  are  hollow  (because  the  heart-wood  has  rotted  or 
been  burned  away),  and  yet  >hey  appear  to  be  healthy  trees? 

(D  or  L)  Split  blocks  from  stems  of  various  kinds  of  trees  (espe- 
cially oak,  chestnut,  beech,  ash,  pine)  through  the  center  and  plane 
the  cut  surfaces.  The  section 
thus  made  is  radial.  Parallel  to 
the  cut  surface  of  one  of  the 
halves  plane  off  the  bark  until 
some  heart-wood  is  exposed. 
This  surface  will  be  a  tangential 
section.  In  the  radial  section  the 
pith-rays  will  appear  as  glossy 
plates  of  wood,  and  in  the  tan- 
gential section  it  will  be  evident 
that  each  ray  extends  some  dis- 
tance up  and  down  the  stem. 


a  b  c 

FIG.  47.  a,  transverse  section  ;  6, 
radial;  c,  tangential.  (U.S. Bureau 
of  Forestry.) 


Evidently  the  delicate  radiating 
lines  or  rays  seen  in  the  trans- 
verse section   were   simply   the 
edges  of  flattened  plates  which  are  placed  vertically  and  radially  in 
the  stem. 

Make  diagrams  showing  structure  seen  in  radial  and  transverse 
sections. 

(D  or  L)  Examine  the  inner  and  outer  layers  of  bark  in  various 
woody  stems.  Soften  pieces  of  bark  by  long  soaking  and  boiling  in 
water,  tear  it  into  pieces  and  observe  the  position  and  direction  of 
the  fibers.  Some  of  these  are  bast-fibers  (elongated  cells).  The 
bast-fibers  of  many  plants  are  of  great  value  in  the  textile  industries, 
e.g.,  flax  (linen),  jute,  ramie,  and  hemp  are  all  obtained  by  rotting 
the  bark  of  certain  plants  and  then  separating  the  bast-fibers. 


172 


APPLIED  BIOLOGY 


FIG.  48.  Half  of  transverse  section  of 
dicotyledon  stem  (Aristolochia).  p, 
pith  (white  background  of  figure)  ; 
seven  fibro-vascular  bundles  (v) ;  cam- 
bium (c)  ;  e,  epidermis.  All  outside 
the  cambium  is  bark,  its  middle  region 
(6)  is  composed  of  hard  bast-fibers. 
(Modified,  from  Strasburger.) 


Some  of  the  fibers  found  in  the 
bark  are  sieve-tubes,  which 
have  already  been  mentioned 
(§103)  as  the  path  of  sap  down 
stems. 

164.  Microscopic  Structure 
of  Dicotyledon  Stems. —  (D)  Ex- 
amine one-,  two-  and  three-year 
stems  in  very  thin  transverse 
sections.  Identify  the  parts 
which  can  be  seen  without  a 
microscope,  and  then  note  the 
appearance  of  the  cells  compos- 
ing them.  Compare  sections 
seen  with  the  microscope  and 
Fig.  48.  A  cross  section  of  a 
very  young  dicotyledon  stem  (e.g.,  bean  or  geranium)  consists  of 
thin-walled  cells  (pith-cells)  and  fibro-vascular  bundles ;  but  the 
bundles,  instead  of  being  scattered  irregularly  as  in  corn  stem  (Fig. 
44),  are  arranged  in  a  circle  (Fig. 
48).  In  the  very  young  stem  the 
bundles  in  the  circle  are  widely 
separated  by  many  pith-cells  ;  but 
soon  new  bundles  appear  between 
the  old  ones,  thus  completing  the 
circle  and  forcing  the  pith  into  thin 
plates  (pith-rays  or  medullary  rays) 
lying  between  the  bundles  (Fig. 
46).  If  a  single  bundle  be  exam- 
ined, it  will  be  seen  to  consist,  as 
in  the  corn ,  of  wood-cells  and  large 
tubes  on  the  inner  side,  and  sieve- 
tubes  and  wood-  or  bast-cells  on 
the  outer  side  (Fig.  49) ;  but  un- 
like the  monocotyledon  (corn) 
bundle,  the  dicotyledon  bundle  has 
between  these  two  regions  a  layer  FIG.  49. 
of  cells  (cambium)  which  are  ca- 
pable of  dividing,  forming  new 
bark  (consisting  of  bast-  and  sieve- 
tubes)  on  the  outer  side  and  new 
wood-cells  and  tubes  on  the  inner 


Transverse  section  of  vas- 
cular bundle  of  a  dicotyledon. 
Lower  part  of  figure  is  towards 
center  of  stem.  /,  woody  fibers 
which  surround  the  bundle;  s, 
sieve-tubes  mingled  with  other  cells; 
d,  wood-ducts;  c,  cambium  layer. 


STUDIES  OF  SEED-PLANTS 


173 


side.     It  is  in  this  way  that  the  new  rings  of  wood  and  bark  are 
formed  in  the  dicotyledonous  stem. 

Longitudinal  sections  should  be  examined  with  the  microscope 
and  compared  with  Fig.  50,  which  will  serve  to  identify  the  parts  seen. 


FIG.  50.  Upper  figure  repres9nts  a  transverse  section  of  a  fibro-vascular 
bundle  of  moonseed.  Bast  is  at  left  of  2,  wood  is  at  right.  Lower  figure  is 
a  longitudinal  section  of  a  sunflower  bundle.  Cambium  at  2  in  upper  figure, 
c,  in  lower  one  ;  d  and  g,  wood-tubes  ;  h,  wood  cells  between  tubes  ;  s,  sieve- 
tubes  ;  p  and  bp,  parenchyma  or  pith  ;  6,  bast.  (From  Bailey,  lower  figure 
after  Wettstein.) 

165.  Functions  of  Parts  of  Stem.  —  Wood-cells  and  bast- 
fibers  are  very  rigid  cells  because  of  their  thick  walls,  and 
thus  are  able  to  give  the  support  which  is  so  necessary  if  a 
plant  is  to  grow  very  large.  They  also  serve  as  a  protection 
to  the  tubes  and  more  delicate  cells  which  they  surround. 


174  APPLIED  BIOLOGY 

Wood-tubes  and  Sieve-tubes.  —  The  tubes  in  the  wood  con- 
duct the  materials  collected  by  the  roots  up  to  the  leaves; 
and  the  sieve-tubes  of  the  inner  bark  distribute  the  food 
materials  prepared  by  the  leaves  to  various  parts  of  the 
plant  body  below  the  leaves. 

Epidermis  and  Corky  Layer  of  the  Bark.  —  These  cells  are 
protective  in  function.  They  prevent  mechanical  injuries 
to  the  delicate  cells  beneath,  and  they  protect  against  loss 
of  water  by  evaporation,  and  against  sudden  changes  of  tem- 
perature. 

Parenchyma  or  Pith  of  the  Bark.  —  In  young  stems  this 
contains  chlorophyll  and  is  able  to  manufacture  starch,  but 
in  older  stems  it  serves  merely  as  a  storehouse  for  foods. 

Cambium.  —  The  region  in  which  all  the  new  cells  are 
formed  for  secondary  growth  in  diameter  of  stems. 

Medullary  Rays  or  Pith-rays.  —  The  paths  along  which 
liquids  may  pass  from  the  wood  outward  across  the  stem  to 
the  cambium  and  bark,  and  inward  from  the  bark  to  the 
cambium  and  cells  of  the  wood.  The  cells  of  the  rays  are  also 
important  for  the  storage  of  food.  Some  stems  (oak,  willow, 
hazel,  and  lilac),  if  collected  during  the  winter  and  preserved 
in  alcohol,  will  show,  if  thin  sections  are  made  and  treated 
with  iodine,  that  starch  is  stored  in  the  rays  and  in  the  pith 
of  the  bark.  In  linden  and  birch  stems  the  presence  of  fat 
can  be  demonstrated  in  sections  by  the  use  of  alcana-root 
dissolved  in  70  per  cent  alcohol,  which  stains  fat  red.  In 
still  other  trees  sugar  is  stored  in  the  pith-rays. 

166.  The  Cambium  in  Grafting  and  Healing  Wounds.  — 
The  power  of  growth  of  the  cambium  is  well  illustrated  in 
these  two  processes.  If  two  branches  of  a  tree  be  scraped 
down  to  the  cambium  and  the  wounded  surfaces  brought  to- 
gether, the  rapid  growth  of  cambium  cells  will  cause  a  grafting 
or  union  of  the  two.  Examples  of  this  can  often  be  seen  in 
a  forest  or  orchard,  for  branches  often  grow  so  near  together 
that  the  swaying  by  the  wind  wears  away  the  bark,  exposes 


STUDIES   OF  SEED-PLANTS 


175 


the  cambium,  and  allows  the  two  branches  to  grow  together. 
Sometimes  two  trees  may  so 
grow  together  when  they  are 
small. 

The  observation  that  stems 
will  grow  together  or  natu- 
rally graft  probably  led  to 
artificial  grafting  of  fruit 
trees.  Essentially  all  graft- 
ing consists  in  bringing  the 
cambium  layer  of  a  piece  of 
twig  with  a  bud  (scion)  from 
one  plant  into  contact  with 
the  same  layer  on  stem  of  an- 
other plant  of  the  same  or 
sometimes  a  closely  related 
species.  Various  ways  of 
doing  this  are  illustrated  in 
Fig.  51.  Grafts  are  usually 
placed  on  young  plants  near 
the  roots  ("  root-grafting")  so 
that  after  the  graft  grows  and 
the  original  stem  is  cut  away, 
only  the  variety  of  the  graft 
will  be  represented  in  the 
stem  above  ground.  Old 
trees  are  grafted  by  cutting 
off  branches ("  top-grafting") 
and  inserting  scions  or  grafts. 
If  a  new  branch  grows  below 
the  graft,  it  will  bear  fruit  of 
the  original  variety. 

Budding  is  essentially  the 
same  as  grafting,  but  a  single 
bud  with  a  slice  of  bark  (Fig. 


FIG.  51.  Upper  row  —  whip-  or 
tongue-grafting,  a,  stem  or  stock  ; 
6,  scion  or  graft ;  c,  united  and 
wrapped.  Middle  row  —  cleft 
grafting,  a,  scion  ;  6,  two  scions 
inserted  in  cleft ;  c,  waxed.  Lower 
row  —  budding.  a,  stem  cut  to 
receive  bud  (6);  c,  bud  inserted 
beneath  bark  ;  d,  wrapped.  Note 
in  all  the  figures  that  cambium  of 
scion  and  stock  meet.  (U.S.  Dept. 
Agriculture.) 


176  APPLIED  BIOLOGY 

51)  is  placed  in  contact  with  the  cambium  of  another  tree,  and 
after  the  bud  grows  into  a  stem  the  original  stem  is  cut  away. 

The  growth  of  new  tissue  which  takes  place  in  the  healing 
of  wounds,  as  after  pruning  or  breaking  of  limbs  by  winds,  is 
essentially  the  same  as  in  grafting.  Examine  trees  from 
which  limbs  have  been  cut  and  notice  the  ring  of  new  tissue 
(callus)  forming  at  the  bark  and  tending  to  grow  over  the 
entire  wound.  Look  for  cases  where  the  wound  is  almost 
overgrown,  and  others  which  are  completely  so.  Look  for 
cases  where  limbs  have  been  broken  or  cut  off  some  distance 
from  the  main  stem,  leaving  a  stub ;  and  compare  the  healing 
in  such  cases  with  those  in  which  the  branch  has  been  cut  off 
near  the  main  branch  or  trunk.  How  should  branches  be 
cut  when  pruning  trees?  Numerous  illustrations  of  these 
points  may  be  seen  in  orchards  and  in  shade  trees. 

In  cases  where  very  large  limbs  have  been  cut  off,  there  is 
a  tendency  of  the  wood  to  decay  before  the  cambium  has  been 
able  to  form  enough  new  tissue  to  cover  the  wound  completely. 
In  old  and  neglected  orchard  trees  many  holes  are  formed  by 
decay  thus  started,  and  which,  after  many  years,  extends  into 
the  very  heart  of  the  tree,  making  it  unable  to  withstand  the 
force  of  the  wind.  The  remedy  is  (1)  prevention  of  decay 
by  painting  or  tarring  all  injured  places  so  as  to  keep  out  the 
bacteria  and  molds  which  cause  decay;  and  (2)  if  decay  has 
already  started,  the  softened  wood  should  be  cut  out  and  the 
cavity  filled  with  tar  or  portland  cement. 

167.  Pruning  is  the  cutting  out  of  unnecessary  and  in- 
jured branches  of  trees,  shrubs,  and  vines.  It  is  necessary 
in  orchards  because  trees  standing  alone  develop  too  many 
branches,  and  grow  too  high  for  convenience  in  spraying 
against  fungi  and  insects  or  in  picking  fruit.  Also,  too  many 
branches  result  in  small  and  imperfect  fruit. 

Pruning  has  been  developed  so  well  that  the  gardener  can 
now  make  a  young  tree  take  almost  any  desired  shape  by 
simply  removing  buds  and  branches  which  are  not  wanted, 


STUDIES   OF  SEED-PLANTS  111 

and  thereby  forcing  growth  in  other  directions.  For  example, 
pruning  off  a  terminal  bud  makes  the  lateral  branches  grow. 
Farmers'  Bulletin  No.  181  should  be  consulted,  and  a  field-trip 
taken  to  some  well-maspaged  orchard  or  tree-nursery. 

Nature's  method  of  pruning  by  crowding  out  superfluous 
branches  is  well  illustrated  in  any  dense  woods.  The  shaded 
branches  soon  die  and  decay,  and  the  result  is  the  growth  of 
long  straight  stems  with  living  branches  near  the  top  only. 
Why  are  trees  set  close  together  in  young  forest  plantations  ? 

Pruning  trees  and  shrubs  at  planting  time  is  of  great  value, 
in  that  it  reduces  the  number  of  leaves  until  roots  can  get 
well  established  in  the  soil.  Unless  the  leaf-surface  is  thus 
reduced,  evaporation  is  likely  to  be  greater  than  can  be  re- 
placed by  absorption  of  water  by  the  roots.  It  is  difficult 
to  transplant  evergreens,  because  pruning  would  spoil  the 
form  of  these  ornamental  trees,  and  the  leaves  cause  excessive 
evaporation  before  new  roots  are  established  in  the  soil. 

168.  Duration  of  Life  of  Plant  Stems.  —  It  is  a  matter  of 
common  observation  that  some  plants  live  only  one  year  and 
that  others  live  many  years.  All  plants  of  which  the  life  of 
a  generation,  beginning  with  the  seed  and  ending  with  death 
of  stem  and  roots,  comes  within  a  year  are  called  annuals. 
That  is  to  say,  an  annual  plant  is  one  which  completes  its  life- 
history  by  starting  from  seed,  producing  seed  and  dying,  root 
and  all,  within  a  single  year.  Some  plants  (e.g.,  beet,  cab- 
bage, thistle,  mullein)  start  from  seed  one  year,  the  roots 
and  base  of  the  stem  live  over  the  winter,  and  in  the  second 
summer  they  blossom,  produce  seed,  and  then  die.  Such  two- 
year  plants  are  called  biennials.  Still  other  plants  (especially 
trees  and  shrubs)  live  more  than  two  years ;  and  such  plants 
are  perennials.  Some  perennials  have  a  soft  stem  which  can- 
not withstand  the  winter,  but  the  underground  parts  are 
hardy  and  new  stems  form  above  ground  in  each  growing 
season.  Examples  are  peony,  golden-rod,  hollyhock,  as- 
paragus, violet,  dandelion,  buttercup,  and  most  common 


178  APPLIED  BIOLOGY 

* 

grasses  of  our  lawns  and  meadows.  Such  plants  are  called 
herbaceous  perennials.  Many  of  them  are  short-lived; 
for  example,  farmers  know  that  the  best  crop  of  red  clover 
is  grown  in  the  first  and  second  years,  and  after  that  the  roots 
are  unable  to  produce  luxuriant  growth  of  stem  and  leaves 
and  gradually  die  out,  giving  place  to  grasses  and  weeds. 
There  are  many  such  cases  among  plants  grown  for  their 
flowers. 

Cultivation  has  changed  the  duration  of  life  of  some  plants. 
A  plant  which  is  a  perennial  in  the  tropical  regions  may  be 
an  annual  in  a  country  with  cold  winters.  Go  into  any 
vegetable  garden  when  the  frost  comes  and  you  will  see  many 
plants,  such  as  tomato,  pepper,  egg-plant,  which  were  started 
very  early  in  the  spring  and  would  have  lived  much  longer 
if  not  killed  by  frost.  These  are  naturally  warm-climate 
plants,  but  in  the  temperate  zone  the  winter  season  limits 
their  duration  to  a  single  summer.  There  are  many  other 
examples  of  changes  in  duration  due  to  cultivation.  Parsnips 
have  been  changed  from  annuals  to  biennials.  Wheat  and 
rye  (both  annuals)  are  commonly  planted  in  September,  live 
over  the  winter,  and  mature  grain  in  July.  Likewise,  gar- 
deners often  start  annual  vegetables  and  "  flowers  "  in  the  late 
autumn,  cover  them  up  over  winter,  and  have  them  mature 
in  early  spring  or  summer.  The  length  of  life  of  such  doubt- 
ful plants  could  be  determined  by  planting  seed  in  very  early 
spring  in  order  to  learn  whether  the  plant  will  mature  seed 
within  a  single  summer.  As  an  example,  certain  kinds  or 
varieties  of  wheat  and  rye  are  planted  in  the  spring  in  the 
extreme  north  (Dakota  and  Manitoba),  where  cold  winters 
would  kill  the  roots  of  the  winter  varieties,  and  this  spring 
wheat  is  harvested  in  the  autumn.  This  could  be  done  where- 
ever  wheat  and  rye  grow ;  but  the  crop  is  usually  not  so  heavy 
as  with  the  variety  planted  in  autumn.  Also,  it  is  of  ad- 
vantage to  the  farmer  to  be  able  to  plant  wheat  and  rye  on 
the  same  ground  after  corn  is  harvested  in  the  autumn, 


STUDIES   OF  SEED-PLANTS  179 

because  proper  cultivation  of  corn  has  left  the  soil  in  con- 
dition for  planting  wheat  without  replowing,  and  also  because 
in  the  corn-growing  regions  oats  and  corn  must  be  planted  in 
the  spring  and  there  would  be  little  time  for  planting  spring 
wheat  and  rye.  Still  another  example  :  many  of  our  garden 
biennials  (carrots,  beets,  cabbage,  onion,  turnips)  will  bloom 
and  form  seed  in  one  long  season  in  a  warm  climate,  or  some- 
times if  forced  with  rich  soil  and  plenty  of  water.  The  above 
are  simply  examples  taken  from  a  long  list  of  cultivated  plants 
whose  duration  of  life  man  has  been  able  to  change. 

Home-work  :  Make  lists  of  the  common  plants  you  know,  classify- 
ing them  as  annuals,  biennials,  or  perennials. 

169.  General  Functions  of  Stems.  —  The  primary  work  of 
ordinary  stems  is  (1)  support  of  the  leaves  in  positions  adapted 
to  their  work  of  food-making  and  transpiration,  (2)  conduc- 
tion of  materials  between  roots  and  leaves,  and  (3)  breathing. 

(1)  In  its  work  of  support  stems  make  use  of  the  elasticity 
and  rigidity  of  woody  tissues  and  also  of  turgidity  due  to 
water  in  the  cells.     A  very  young  plant  depends  largely  upon 
turgidity,  for  if  water  is  withheld  from  the  soil,  the  leaves  and 
stems  become  wilted.     A  young  Windsor-bean  plant  growing 
in  a  pot  will  illustrate  this.     If  the  soil  be  allowed  to  dry,  the 
stem  will  lose  its  turgidity  (become  wilted)  and  will  gradually 
bend  over  until  the  leaves  touch  the  soil.     If  then  water  is 
added  to  the  soil,  the  stem  will  become  turgid  and  rigid  with 
remarkable  rapidity,  and  within  two  or  three  hours  will  be  as 
erect  as  ever.     Stems  of  older  plants  of  most  species  do  not 
lose  their  rigidity  when  water  is  wanting,  for  woody  tissues 
furnish  the  necessary  rigidity.     A  straw  of  wheat  or  rye  is  a 
splendid  example  of  rigidity  due  to  woody  tissues  arranged 
in  cylindrical  form. 

(2)  The  work  of  conducting  materials  is,  as  we  have  seen 
in  the  bean  plant,  due  largely  to  the  tubes  in  wood  and  bark 
which  are  specially  fitted  to  conducting  liquids  lip  and  down 


180  APPLIED  BIOLOGY 

the  stem.  Other  cells  of  the  stem  also  play  a  part  in  con- 
duction of  liquids,  especially  in  the  transverse  or  horizontal 
direction. 

(3)  Breathing  by  stems  occurs  through  certain  openings  in 
the  outside  flayers  of  tissue.  The  epidermis  of  herbaceous 
stems  has  stomata  similar  to  those  of  leaves.  Examine  with 
microscope  epidermis  from  such  plants  as  bean,  tradescantia, 
and  begonia.  In  plants  with  corky  bark  the  stomata  are 
replaced  by  openings  known  as  lenticles  (meaning  lens-shaped) 
or  stem-pores.  Good  examples  may  be  seen  on  young  stems 
of  cherry  and  plum.  Notice  that  on  the  older  parts  of  these 
stems  the  lenticles  are  elongated  by  growth.  The  holes  in 
cork  used  for  bottles  look  like  holes  made  by  worms,  but  they 
are  lenticles.  Examine  a  bottle  cork  and  note  the  parallel 
holes  which  originally  extended  through  the  thick  corky  bark 
to  the  cambium,  thus  allowing  air  to  penetrate  to  the  active 
cells. 

170.  Special  Adaptations  of  Stems.  —  In  addition  to  the 
primary  functions  discussed  above,  many  stems  have  become 
adapted  or  fitted  to  special  kinds  of  work  such  as  climbing, 
propagation,  living  underground,  and  in  still  other  ways  to  be 
described  in  the  paragraphs  which  follow. 

171.  Propagative  Branches.  —  Strawberries,  red  raspber- 
ries,   currant,    and   gooseberry  are   examples  of  numerous 
kinds  of  plants  which  have  some  branches  either  lying  on 
the  surface  of  the  soil  or  creeping  underground,  and  which 
form  roots  and  develop  new  plants.     When  these  branches  are 
underground,  gardeners  call  the  new  plants  which  come  up 
"  suckers." 

172.  Branches  for  Climbing  :  Tendrils.  —  Slender  branches, 
without  leaves  and  buds,  are  in  many  plants  adapted  for 
climbing.     Examine  specimens  of  tendrils  from  grape-vine, 
Virginia-creeper,    Boston-ivy,  cucumber,  squash-vines,  pas- 
sion-flower   plant,    or   gourd-plants.     Note    that   some   are 
single   tendrils,   some   are  branched,    and    some   have   flat 


STUDIES   OF  SEED-PLANTS  181 

disks  at  the  tips.  If  possible,  observe  the  growth  and 
movements  of  tendrils  on  some  plant  which  can  be  watched 
from  day  to  day. 

Leaves  of  certain  plants,  as  those  of  the  pea,  form  tendrils ; 
but  these  are  easily  distinguished  from  branches  because  they 
are  connected  with  leaves  and  not  in  the 
usual  positions  of  branches. 

173.  Branches  as  Thorns.  —  Examina- 
tion of  specimens  of  thorns  from  trees  of 
pear,  honey-locust,  hawthorn,  buck-thorn, 
and  others  shows  that  they  are  branches, 

or  tips  of  branches.  Prickles  on  many  F  52  ^eaf  and 
plants  are  simply  elevations  of  the  bark.  stem-tendrils  (/•) 
Peel  off  some  bark  from  stems  of  blackberry  of  Ampeiopsis 
or  rose  and  note  that  the  prickles  are  not 
attached  to  the  wood  as  branches  of  stems  are.  Barberry 
prickles  are  modified  leaves,  and  those  of  black  locust  are 
stipules  of  leaves. 

The  uses  of  thorns  and  prickles  are  sometimes  doubtful. 
While  they  appear  to  be  adaptations  for  protection  against 
grazing  animals,  it  is  well  known  that  often  they  do  not  so 
protect.  Cows  and  sheep  often  eat  the  thorny  branches, 
especially  young  and  delicate  shoots  when  the  thorns  or 
prickles  are  soft.  Certainly  these  sharp  structures  are  not 
absolutely  necessary,  for  many  plants  without  them  appear 
to  flourish  as  well  as  their  "  armed  "  relatives.  It  is  probable 
that  prickles  and  thorns  developed  first  without  any  ref- 
erence to  defensive  use  (e.g.,  the  spines  of  cacti,  which  are 
leaves  reduced  in  adapting  to  dry  climate),  and  that  now 
they  may  sometimes  be  of  slight  advantage  to  the  individual 
plants  that  possess  them. 

174.  Twining  Stems.  —  Some  stems  climb  without  the  aid 
of  tendrils.     Study  plants  of  pole-beans,  morning-glory,  hop, 
on  any  wild  plants  available.     Lay  your  watch  on  the  ground 
with  the  hour-mark  pointing  north,  and  note  whether  plants 


182  APPLIED  BIOLOGY 

observed  twine  in  the  direction  the  watch-hands  move  (i.e., 
clockwise,  west-north-east) ;  or  in  opposite  direction  (counter- 
clockwise, east-north-west).  Plants  of  some  species  appear 
to  move  always  in  one  of  these  directions. 

175.  Creeping  Stems.  —  In  walking  across  pastures,  sandy 
fields,  and  other  places  where  tall  plants  do  not  grow,  look 
for  plants  which  have  their  stems  prostrate  on  the  ground. 
Do  you  see  that  such  plants  have  any  advantage  over  erect 
tall  stems  under  the  conditions  in  which*they  live? 

176.  Underground  Stems. — Many 
plants  have  their  main  stems  under- 
ground. Three  common  forms  of 
such  stems  are  rootstocks,  tubers,  and 
bulbs. 

Rootstocks.  —  These   are   root- 

FIG.    53.      Underground     1-1  i_-    i_  j.u  *i 

stem  of  Solomon's  seal.      llke    stems    whlch    Srow    m    the    SOlL 

a,  terminal  bud ;  b,  c,  d,  Many  ferns,  grasses,  sweet  flag,  golden- 
ye  "Troote'  '""^  rod'  quack-grass,  peppermint,  iris, 
Solomon's  seal,  and  trilliums  are  ex- 
amples of  common  plants  with  rootstocks  (rhizomes).  That 
these  are  not  true  roots  is  shown  by  the  presence  of  buds 
and  by  the  formation  of  branches  and  leaves.  Solomon's  seal 
(Fig.  53)  is  especially  good  for  study.  Its  rootstock  sends  up 
an  erect  branch  every  spring  which  becomes  the  above-ground 
stem  with  leaves.  All  the  above-ground  part  of  the  plant 
is  herbaceous  and  dies  in  autumn.  The  seal-like  scars  on  the 
rootstock  mark  the  positions  of  the  above-ground  branches 
in  successive  years.  The  oldest  portion  of  the  rootstock 
dies  and  decays ;  and  some  plants  have  short  rootstocks,  be- 
cause the  part  more  than  a  few  years  old  has  died  and  decayed. 
Tubers.  —  Many  of  the  rootstocks  mentioned  above  store 
food,  but  do  not  undergo  noticeable  thickening  at  any 
particular  point.  In  some  plants  storage  of  food  in  stem 
underground  causes  great  enlargement  and  produces  tubers 
(e.g.,  the  common  potato  and  Jerusalem  artichoke). 


STUDIES  OF  SEED-PLANTS 


183 


(L)  Examine  a  tuber  of  potato,  or  artichoke.  Note  the  point 
where  it  was  attached  to  the  main  stem  of  the  plant.  Examine  the 
"eyes,"  which  are  buds.  Each  "eye"  is  capable  of  developing  a  new 
plant,  and  hence  to  avoid  having  too  many  plants  in  one  place 
gardeners  cut  tubers  into  pieces,  each  having  two  or  three  "eyes." 
Dig  up  a  potato  plant,  and  note  position  of  the  old  and  the  new 
potatoes. 

Bulbs  are  short  and  greatly  enlarged  underground  stems, 
producing  stalks  and  leaves  from  the  upper  surface  and  roots 
from  the  lower.  The  onion  is  a  good  example  of  a  bulb.  A 
longitudinal  cut  through  the  center  shows  it  to  be  a  short 
stem  surrounded  by  the 
thickened  layers,  which  are 
modified  leaves.  Indian  tur- 
nip (Jack-in-the-pulpit)  and 
crocus  have  similar  short 
and  bulb-like  stems,  but 
they  are  solid  and  not  com- 
posed of  layers  like  the 
onion.  Such  solid  short  un- 
derground stems  are  often 
called  corms;  but  there  is 
no  sharp  distinction  and 
corms  are  often  called  solid 
bulbs. 

"Stemless"  Plants.— 
Many  common  plants  (e.g., 
dandelions,   plantains)   have  short  stems  and  are  often  in- 
correctly said  to  be  "  stemless." 

177.  Condensed  Stems  above  Ground.  —  Tubers  and 
bulbs  have  been  mentioned  as  examples  of  condensed  under- 
ground stems.  The  various  forms  of  cacti  are  condensed 
stems  above  ground.  The  flattened  leaf-like  parts  of  prickly- 
pear  cactus  are  not  leaves,  but  branches  of  the  stem.  The 
thorns  are  reduced  leaves.  The  flat  and  thickened  branches 
not  only  perform  the  ordinary  work  of  leaves,  but  are  also 


FIG.  54.  Potato  plant,  developed  from 
dark-colored  tuber  in  center.  New 
tubers  are  thickenings  of  underground 
branches  of  the  stem.  (From  Stras- 
burger.) 


184  APPLIED  BIOLOGY 

important  reservoirs  of  water  needed  in  times  of  drought. 
The  absence  of  leaves  gives  the  advantage  of  greatly  reduced 
surface  exposed  to  evaporation. 

178.  Stems    Adapted    as    Leaves.  —  In    extremely   dry 
countries  some  plants  have  formed  the  habit  of  producing 
very  small  leaves  or  none  at  all,  in  order  that  there  may  be  as 
little  loss  of  water  as  possible.     An  extreme  case  of  this  kind 
is  seen  in  cacti,  described  in  the  preceding  section.     Our 
garden  asparagus  is  by  nature  adapted  to  very  dry  con- 
ditions.    Its  only  leaves  are  the  scales  which  appear  on  the 
young  edible  shoot  early  in  the  spring,  and  the  mass  of  slen- 
der green  structures  which  develop  later  in  the  summer  are 
branches.     In  the  greenhouse  plant  commonly  called  "  smi- 
lax,"  the  flattened  leaf-like  structures  are  stems.     This  is 
evident  from  their  position  in  the  axils  of  the  very  small 
scaly  leaves.     Such  modified  stems  are  called  cladophylls, 
meaning  branch-leaves. 

That  this  modification  and  reduction  of  leaf  surface  is  an 
adaptation  to  environment  brought  about  by  external  in- 
fluences is  proved  by  the  fact  that  certain  plants  which 
produce  leaves  in  regions  of  moderate  water-supply  become 
very  much  like  spiny  cacti  when  grown  in  desert  conditions. 

179.  Economic  Value  of  Wood  Structure.  —  The  useful- 
ness and  consequent  monetary  value  of  lumber  depends  upon 
the  nature  of  the  various  structures  seen  in  the  sections  of 
stems  (§  163).     For  many  purposes  hardness  and  strength 
combined  are  desired  (e.g.,  white  oak  for  wagon-axles,  hickory 
for    ax-handles    and    wheel-spokes).     Sometimes    a    light, 
elastic,  straight-grained  wood  is  wanted ;   and  so  ash  is  most 
common  for  the  long  handles  of  hoes,  shovels,  pitch-forks, 
etc.     Woods  which  resist  decay  because  they  contain  resins, 
oils,  or  other  protecting  substances  are  needed  for  fence-posts 
and  telephone  poles ;  and  for  such  purposes  cedar,  chestnut, 
oak,  mulberry,  and  black  locust  are  usually  preferred.     White 
and  red  cedar,  cypress,  and  redwood  make  the  best  shingles 


STUDIES   OF  SEED-PLANTS  185 

now  in  the  market.  For  some  purposes  wood  softer  than  oak 
is  desirable  because  easy  to  work  —  carpenters  charge  much 
more  for  working  oak  than  for  soft  woods  like  white  pine. 
Hence  pine,  hemlock,  spruce,  redwood,  basswood,  tulip-wood 
or  whitewood,  cedar,  sweet  gum,  and  poplar  are  used  for 
numerous  purposes  where  soft  and  light  wood  will  serve. 
For  high-grade  furniture,  woods  with  beautiful  grain  and 
color  combined  with  hardness  are  desired;  and  hence  the 
popularity  of  mahogany,  black  walnut,  rosewood,  ebony,  and 
oak. 

It  often  happens  that  in  a  given  locality  a  wood  is  used 
for  a  purpose  because  it  is  the  cheapest  available,  but  not  the 
best.  For  example,  hemlock  is  largely  used  in  Eastern  states 
for  frames  of  buildings  ;  but  its  softness  and  tendency  to  split 
makes  it  far  inferior  to  spruce  among  soft  woods  and  not  to 
be  compared  with  the  oak,  formerly  much  used  for  this  pur- 
pose. White  pine  and  spruce  are  often  used  for  cheap  floors, 
but  are  as  much  inferior  to  yellow  pine  as  this  is  to  hard 
maple  and  oak  for  this  purpose.  Such  semi-hard  woods  as 
elm,  chestnut,  birch,  and  basswood  are  now  abundant  in 
cheap  furniture,  and  are  often  stained  to  imitate  harder 
woods  like  oak  and  mahogany. 

Woods  like  chestnut  and  red  oak,  with  large  wood-ducts, 
making  the  annual  rings  very  conspicuous,  are  said  to  be 
"  open-grained."  In  finishing  such  woods  for  furniture  it 
is  first  necessary  to  use  wood-fillers,  which  are  pastes  for 
filling  the  open  spaces  left  after  planing,  and  thus  give  a 
perfectly  level  surface  to  receive  the  varnish.  Examine 
polished  oak  furniture  and  compare  with  an  oak  board  which 
has  been  planed  but  not  filled. 

A  wood  which  may  be  split  into  straight  pieces  parallel 
to  the  central  axis  of  the  stem  is  said  to  be  "  straight  grained." 
Ash,  chestnut,  and  hemlock  are  commonly  so  ;  but  irregularity 
in  thickness  of  annual  rings  and  in  position  of  the  medullary 
rays,  vascular  bundles  and  knots  makes  some  woods  more  or 


186  APPLIED  BIOLOGY 

less  "cross-grained";  that  is,  they  tend  to  split  obliquely 
and  not  parallel  to  the  central  axis  of  the  stem.  Many  woods 
(such  as  elm,  sycamore,  apple)  are  often  difficult  to  split 
because  their  wood-fibers  are  crossed  and  interlaced.  Split 
some  pieces  of  boards  or  branches  of  various  trees  by  driving 
a  wide  chisel  carefully  and  notice  the  direction  of  the  fibers 
of  the  wood. 

The  usefulness  of  boards  for  certain  purposes  depends  upon 
the  direction  of  sawing.  Examine  pieces  of  pine,  cypress, 
maple  and  other  boards,  especially  where  they  have  been 
subjected  to  wear,  as  in  floors,  and  note  the  direction  of  the 
cut  of  boards  which  have  splintered  or  in  other  ways  become 
undesirable.  The  transverse  section  is  best  if  a  block  is  to  be 
subjected  to  great  strain,  as  in  pavement-blocks,  mallets,  etc. 

The  decorative  value  of  the  grain  of  woods  depends  in  part 
upon  the  way  the  logs  are  sawed  into  boards.  Compare  the 
tangential  and  radial  sections  of  oak  and  other  woods,  and 
decide  which  is  the  most  beautiful  cut.  Most  people  prefer 
the  radial  cut  of  oak  with  the  exposed  glossy  medullary  rays 
capable  of  excellent  polish.  In  the  usual  method  of  cutting 
boards  only  a  few  boards  near  the  center  of  the  log  will  be 
radial  sections  or  nearly  so.  All  the  others  will  be  more  or 
less  tangential  (see  4,  5,  and  6  in  Fig.  55).  In  sawing  oak 
and  other  woods  with  prominent  rays  for  furniture  and  house- 
finishing  it  is  best  to  saw  as  shown  in  Fig.  56.  Two 
boards  (sometimes  four)  are  taken  from  the  middle  (cuts 
1,  2).  Next  the  same  number  of  boards  are  taken  from  the 
middle  of  each  half  (3,  4).  These  will  show  the  same  grain 
as  the  first  boards  cut,  but  will  be  less  than  half  as  wide. 
Then  each  of  the  "  quarters  "  of  the  log  is  taken  separately 
and  sawed  into  boards  cut  as  radially  as  possible.  There 
are  several  possible  ways  of  sawing  "quarters,"  but  a  little 
study  of  the  diagram  and  of  blocks  of  wood  will  show  that 
cutting  as  in  A  and  D,  Fig.  56,  is  to  be  preferred  for 
"  quarter-sawing  "  the  choicest  woods,  because  it  will  re- 


STUDIES   OF  SEED-PLANTS 


1ST 


suit  in  the  largest  number  of  radial  sections  showing  the 
pith-rays. 

In  some  woods  the  most  beautiful  sections  are  tangential. 
In  order  to  get  the  greatest  number  of  such  sections,  such 
woods  are  often  cut  into  sheets  of  veneer  by  means  of  a 
machine  with  a  large  knife  which  cuts  an  immense  shaving, 


FIG.  55.  Ordinary  sawing  of  lumber.  FIG.  56.  Four  methods  of  quarter 
sl,  s2,  s3,  slabs  removed  by  cuts  1,  sawing.  Cuts  1,  2,  3,  4  remove 
2,  3  ;  and  followed  by  cuts  4  boards  which  are  perfect  radial  see- 
to  12.  tions  on  surfaces  1  and  3.  Then 

each  "  quarter  "  is  sawed  in  order 
5,  6,  7,  8.  The  method  illustrated 
in  "quarter  "  C  is  least  desirable. 

a  continuous  tangential  section,  as  the  log  is  revolved  on  its 
axis.  In  this  way  an  entire  log  can  be  cut  into  tangential 
section.  Pieces  of  the  sheet  of  veneer  are  then  glued  on  the 
surface  of  other  and  less  valuable  boards.  This  method  also 
has  the  advantage  of  avoiding  the  waste  of  material  which 
results  from  cutting  with  saws. 

180.  Forestry.  —  The  application  of  scientific  principles 
to  the  management  of  forests  is  known  as  forestry.  Such 
scientific  management  has  become  necessary  because  the  use 
of  tree  stems  as  lumber  has  greatly  reduced  the  extent  of 
forests ;  and  in  the  interest  of  future  supply  it  is  now  desirable 


188  APPLIED  BIOLOGY 

that  forest  planting  and  forest  conservation  be  practiced  ex- 
tensively. And  not  only  is  it  important  that  forests  should 
be  preserved  and  new  ones  developed  for  the  sake  of  a  con- 
tinued supply  of  lumber;  but,  also,  among  many  other  things 
which  make  forests  valuable  are  their  effect  upon  climate, 
prevention  of  soil  erosion,  prevention  of  sudden  floods, 
aesthetic  value,  and  affording  protection  for  birds  and  other 
desirable  animals.  Each  of  these  is  important  enough  to 
warrant  much  attention  to  forestry ;  and  recognition  of  such 
facts  is  responsible  for  the  general  awakening  of  interest  in 
preservation  and  improvement  of  American  forests.  The 
United  States  government  now  maintains  at  Washington 
a  bureau,  known  as  the  Forest  Service,  which  has  charge  of 
the  national  forest  reserves,  and  which  also  encourages  private 
work  in  forestry  by  publishing  pamphlets  and  by  giving 
advice.  Among  the  best  of  such  pamphlets  now  available 
are  the  "  Primer  of  Forestry  "  by  Pinchot,  which  is  in  two 
parts  (free  to  those  who  apply  to  the  Department  of  Agri- 
culture) ;  and  many  leaflets  (also  free)  giving  information 
as  to  how  to  plant  special  kinds  of  trees.  The  "  Primer  of 
Forestry  "  should  be  obtained  and  read  by  all  students  of 
biology.  Another  interesting  book  in  the  same  line  is  Roth's 
"First  Book  of  Forestry." 

LEAVES   OF  SEED-PLANTS 

181.  Functions  of  Leaves.  In  connection  with  the  study 
of  the  bean  plant  three  functions  of  leaves  (breathing,  trans- 
piration, and  starch-making)  have  been  mentioned;  In 
addition  to  these  functions,  which  belong  to  all  typical  leaves, 
some  plants  have  leaves  fitted  for  special  functions,  which  will 
be  described  in  the  paragraph  on  adaptations  of  leaves. 

In  order  to  carry  on  most  efficiently  the  three  main  func- 
tions, it  is  important  that  leaves  should  be  arranged  so  as  to 
have  the  most  favorable  exposure  to  light  and  air.  This 


STUDIES   OF  SEED-PLANTS  189 

requires  (1)  great  expanse  of  surface,  which  is  secured  by  the 
broad  blades ;  and  (2)  arrangement  so  that  they  will  shade 
each  other  as  little  as  possible.  This  latter  point  deserves 
some  special,  study. 

182.  Arrangement  of  Leaves.  —  One  of  the  most  im- 
pressive points  about  leaves  is  their  position  on  the  branches 
of  plants.  It  would  be  a  careless  observer  indeed  who  does 
not  notice  that  the  leaves  are  commonly  arranged  in  rows, 
but  perhaps  only  the  student  of  botany  observes  that  there 
may  be  two,  three,  four,  five  and  even  more  rows.  This  has 
been  already  indicated  in  the  study  of  twigs,  on  which  the 
buds  mark  the  positions  of  future  leaves  (§  156).  The 
student  should  look  at  plants  having  different  numbers  of 
rows,  viewing  the  main  stem  from  above,  and  note  how  the 
number  of  rows-  affects  the  shading  of  leaves  by  each  other. 

The  length  of  the  internodes  of  stems  affects  light  exposure 
by  separating  the  leaves.  If  the  leaves  are  close  together, 
it  is  evident  that  they  will  tend  to  shade  each  other. 

The  size  of  leaves  and  the  number  of  leaves  have  a  definite 
relation  to  the  arrangement  in  many  plants.  Some  plants 
have  a  few  large  leaves,  while  others  have  many  small  leaves. 
If  possible,  contrast  such  trees  as  catalpa  and  willow,  or 
hickory  and  birch,  looking  at  small  trees  of  these  species  from 
above  and  at  large  trees  from  below. 

Varying  length  of  branches  of  the  stem  and  of  petioles  of  most 
leaves  of  trees  aid  in  making  the  best  adjustment.  Numerous 
plants  illustrate  this.  The  lower  branches  of  most  trees  are 
longest  and  so  hold  their  leaves  out  of  the  shadow  of  the  upper 
branches ;  and  also  some  of  the  leaves  have  long  petioles  so 
that  their  blades  avoid  shading  by  leaves  above. 

Twisting  of  stems  and  petioles  brings  some  leaves  into  the 
best  position.  This  is  most  strikingly  illustrated  by  the 
horizontal  branches  as  contrasted  with  vertical  branches  on 
the  main  stem  of  the  same  trees. 

Most  leaves  face  the  source  of  light,  as  may  be  seen  in  plants 


190  APPLIED  BIOLOGY 

growing  in  window-boxes ;  but  many  grasses  and  other  plants 
have  leaves  nearly  vertical  or  erect,  and  others  hold  their  leaves 
edgewise.  In  such  cases  light  does  not  strike  the  surface 
directly.  Leaves  so  arranged  are  probably  very  sensitive 
and  the  indirect  light  is  better  suited  to  their  work. 

Climbing  plants  grow  so  as  to  arrange  their  leaves  with  ref- 
erence to  light  as  definitely  as  do  plants  with  erect  stems. 
Many  vines  spread  their  leaves  over  the  outermost  branches 
of  trees,  especially  in  dense  forests.  In  fact,  the  climbing 
habit  is  believed  to  have  its  purpose  in  securing  the  best  pos- 
sible exposure  of  leaves  to  light. 

It  is  interesting  to  note  that  even  plants  adapted  to  grow- 
ing in  the  shade  of  forests  appear  to  have  leaves  arranged  for 
best  exposure  to  light.  This  shows  that  while  such  plants 
are  adjusted  to  shade,  they  must  have  as  much  of  the  diffuse 
light  as  possible  under  the  conditions. 

The  leaves  of  some  plants  attain  the  best  exposure  by  move- 
ments, such  as  have  been  mentioned,  adjusting  their  leaves 
to  the  amount  of  light  best  suited  to  them. 

Finally,  the  forms  of  leaves  appear  to  be  of  some  meaning 
in  connection  with  best  exposure  to  light.  The  triangular 
leaves  of  some  plants  seem  to  fit  together  better  than  rounded 
leaves  would  on  the  same  arrangement  of  branches.  The 
numerous  kinds  of  notches,  lobes,  and  branching  of  leaves 
allow  light  to  pass  through  to  lower  leaves.  If  one  examines 
plants  with  such  leaves  exposed  to  bright  sunlight,  it  is  evi- 
dent that  some  light  passes  through  the  spaces  between  leaves. 

There  are  numerous  other  ways  in  which  certain  plants 
have  their  leaves  arranged  in  order  to  get  the  best  possible 
light.  The  best  for  a  given  plant  does  not  mean  the  most 
light,  for  at  times  sunlight  may  be  so  intense  as  to  be  injurious 
to  some  plants.  However,  this  is  usually  provided  for  by 
movement  in  some  species  and  by  thick  cells  which  reduce 
the  intensity  of  light  that  reaches  the  more  sensitive  middle 
tissues  of  leaves. 


STUDIES  OF  SEED-PLANTS  191 

In  field  trips  taken  in  connection  with  this  course  in  biology, 
attention  should  be  directed  to  the  arrangement  of  leaves  in  the 
light  relation  whenever  good  specimens  are  found.  The  discus- 
sion of  the  above  outline  of  the  most  important  types  of  leaf- 
arrangement  should  be  taken  up  in  the  laboratory  with  specimens  at 
hand  to  illustrate  the  main  points,  and  pupils  should  make  outline 
sketches  showing  various  types  of  leaf-arrangement. 

Reading  for  pupils:  Chapter  II,  "Foliage  Leaves:  the  Light- 
Relation,"  in  Coulter's  "Plant  Relations."  The  same  chapter  is  in 
"Plant  Studies"  and  "Plants"  by  the  same  author.  Or  Chapter 
X  in  Bergen' s  "  Foundations  of  Botany,"  or  Chapter  XI  in  Bergen 
and  Davis's  "Principles  of  Botany." 

183.  Special  Adaptations  of  Leaves.  —  While  the  primary 
purposes  of  leaves  are  starch-making,  transpiration,  and 
breathing,  many  leaves  have  assumed  additional  work  for 
which  their  structure  shows  special  adaptations.  The  most 
interesting  of  these  are  those  concerned  with  catching  insects, 
storing  food,  climbing,  and  protection  against  animals  (leaves 
modified  into  prickles). 

Leaves  as  Insect  Traps.  —  Certain  plants  growing  in 
bogs  have  curious  leaves  adapted  to  catching  insects.  Some 
of  these  plants  have  leaves  in  the  form  of  deep  cups  or  pitchers 
containing  water  in  which  insects  are  drowned  (pitcher 
plants) ;  some  species  (the  sun-dews)  have  strong  bristles 
coated  with  a  sticky  substance  which  holds  fast  any  insects 
which  happen  to  touch  them  (Fig.  57) ;  and  a  third  kind 
(Venus  fly-trap)  have  at  the  end  of  each  leaf  a  folded  structure 
which  opens  and  shuts  very  much  like  a  steel-trap  (Fig.  2). 
In  all  these  cases  the  insects  caught  either  decay  by  action 
of  bacteria  or  are  digested  by  certain  fluids  secreted  by  the 
leaves,  and  are  absorbed  by  the  cells  of  the  plant.  Thus 
the  ordinary  food-supply  of  these  plants  is  supplemented 
by  animal  food,  and  such  plants  are  called  carnivorous 
(flesh-eaters),  or  sometimes  insectivorous  (insect-eaters). 

Leaves  for  Climbing.  —  The  leaf-stalk  or  petiole  acts  as 
a  tendril  in  some  plants  (nasturtium,  clematis,  etc.).  In 


192 


APPLIED  BIOLOGY 


the  compound  leaf  of  the  pea  and  of  many  other  members 
of  the  bean  family  there  is  a  tendril  in  place  of  the  terminal 
leaflet  of  the  compound  leaf  (Fig.  58) .  There  are  some  plants 
which  have  the  tips  of  simple  leaves  serving  as  tendrils. 
In  some  plants  (common  green  brier  or  cat-brier)  a  pair  of 
tendrils  are  attached  at  the  base  of  each  petiole ;  and  these 
are  modified  stipules  of  the  leaf.  It  is  evident  that  any 
part  of  leaves  (petiole,  blade,  or  stipules)  may  be  modified 


FIG.  57.  Modified 
leaf  of  a  sundew. 
The  sticky  bristles 
catch  insects.  (From 
Strasburger.) 


FIG.  58.  Leaf  of  pea.  Large 
stipules  (ri),  leaflets  of  the 
compound  leaf  (6),  terminal 
leaflets  modified  into  tendrils 
(r). 


to  serve  as  tendrils.  It  has  already  (§  172)  been  pointed  out 
that  some  tendrils  are  modified  branches.  A  comparison  of 
position  and  structure  will  usually  make  it  easy  to  decide 
whether  particular  tendrils  are  from  leaves  or  branches  of 
stem. 

Leaves  for  Food-storage.  —  It  has  already  been  stated  that 
the  concentric  layers  of  bulbs  (onion,  tulip,  etc.)  are  the 
bases  of  leaves  thickened  by  food  stored  for  use  in  the  next 
season's  growth.  The  use  of  the  food  becomes  evident  if 
one  plants  an  onion  or  other  bulb  in  a  pot,  for  after  a  time 
the  layers  become  reduced  to  thin  and  dry  sheets.  The 
thick  cotyledons  of  bean,  pea,  acorn,  etc.,  are  also  examples 
of  food-storage  in  leaf -like  structures. 

Leaves  as  prickles  occur  in  some  plants.     Examine  branches 


STUDIES   OF  SEED-PLANTS 


193 


of  common  barberry  and  notice  gradations  between  the 
spiny  leaves  and  the  prickles.  Also  notice  that  buds  are  in 
the  axils  of  the  spines,  and  compare  with 
the  position  of  buds  with  reference  to  leaves 
(§  157). 

Stipules  of  Leaves.  —  These  are  often 
absent,  and  when  present  are  variously 
modified.  Compare  those  of  leaves  of 
clover,  pea,  rose,  India  rubber  tree,  buck- 
wheat. In  some  plants  (e.g.,  pansy)  the 
stipules  are  so  large  that  they  are  not  easily 
distinguished  from  leaves.  The  outside 
bud-scales  in  beech,  tulip-tree,  and  mag- 
nolia are  stipules.  The  prickles  at  base  of 
leaves  of  black  locust  tree  are  obviously 
modified  stipules  (Fig.  60). 

Scale-leaves.  —  This  name  is  applied  to 
the  small  scale-like  structures  which  do  not  have  the  usual 
functions  of  leaves,  but  which  are  chiefly  for  protection  in 
bulbs  and  buds.  The  inner  scales  of  the  horse-chestnut  bud 
have  a  structure  which  suggests  that  scale-leaves  are  modi- 
fied or  reduced  leaves.  The  structures 
termed  bracts  which  are  at  the  base  of 
flower-stalks  (e.g.,  the  petal-like  bracts  of 
the  clusters  of  dogwood  flowers  and  the 
sepal-like  bracts  on  the  hepatica  flowers) 
are  of  similar  origin. 

184.    The  Parts  of  a  Leaf.  —  In  the  study 
of  the  bean  leaf,  the  petiole,  blade,  and 
FIG.  60.    The  spines   stipules  were  mentioned  as  the  parts  of  a 
ole  Jofe°a   b^aVk   typical  leaf;  but  many  leaves  have  some 
locust   leaf    are   of  these   parts    lacking.      Especially    are 

modified  stipules. 


to.  59.  The  leaf 
of  yellow  vetch  is 
reduced  to  a  ten- 
dril (6),  while  the 
two  stipules  (n) 
are  large  and  serve 
as  leaves. 


tinguish  between  petiole  and  blade  (grasses,  lilies,  etc.),  and 
leaves  of  many  species  of  plants  have  no  stipules. 


194  APPLIED  BIOLOGY 

185.  Floral  Leaves.  —  Careful  study  of  the  development 
of  flowers  has  led  botanists  to  regard  the  sepals,  petals, 
stamens,  and  pistil  as  highly  modified  leaf-like  structures 
(§  202),  and  they  are  sometimes  called  floral  leaves. 

186.  Forms  of  Leaves.  —  Students  who  have  not  become 
acquainted  with  the  various  common  forms  of  leaves  should 
compare    them  whenever    an    opportunity   offers.     Special 
names  are  applied  to  the  shapes  of  leaves  and  to  the  different 
kinds  of  notched  and  lobed  margins.     For  these  names  con- 
sult the  glossaries  of  books  for  identifying  plants,  and  the 
figures  in  such  books  as   Gray's   "  Lessons  with   Plants." 
Such  names  are  not  worth  memorizing  unless  one  makes 
frequent  use  of  manuals  for  identifying  plants. 

187.  Veins  of  Leaves.  —  The  type  of  veining  found  in 
the  bean  leaf  is  known  as  netted  veining,  while  that  seen  in 
the  corn  plant  is  parallel  veining.     Most  monocotyledons 
have   parallel  veining,  and   most   netted-veined   leaves  are 
found  on  dicotyledons.     The  arrangement  of  the  veins  in 
these  two  types  appears  to  have  nothing  to  do  with  the  work 
of  the  leaves,  for  the  two  kinds  of  leaves  perform  essentially 
the  same  functions.     The  kind  of  veining  is  chiefly  of  interest 
in  classifying  plants,  for  it  at  once  suggests  either  mono- 
cotyledons  or   dicotyledons;    and   this   suggestion   can  be 
tested  by  examining  the  structure  of  the  stem  (§  161)  and 
certain  points  with  regard  to  the  flowers  (§  197). 

188.  Simple  and  Compound  Leaves.  —  The  bean  leaf  has 
been  described  as  compound  because  its  blade  is  divided  into 
separate  parts  or  leaflets.     For  convenience  in  identifying 
and  describing  plants,  botanists  have  termed  those  com- 
pound leaves  with  leaflets  arranged  on  sides  of  the  leaf- 
stalk (Fig.    58),   as   in   honey-locust,    black    locust,    elder, 
ash,  pea,  parsnip,  etc.,  pinnate  (meaning,   arranged  like  a 
feather) ;  while  the  term  palmate  (meaning,  like  a  hand)  is 
applied  to  those  leaves  like  clover,  horse-chestnut,  buck- 
eye, in  which  the  leaflets  are  all  attached  to  the  end  of 


STUDIES   OF  SEED-PLANTS  195 

the  leaf-stalk,  similar  to  the  attachment  of  fingers  to  the 
palm  of  the  hand. 

Either  type  of  compound  leaves  may  have  the  leaflets 
again  divided  one  or  more  times,  as  on  young  shoots  of  honey- 
locust,  in  meadow-rue,  carrot,  and  parsnip.  Compare  carrot 
and  parsnip  leaflets. 

Compare  pinnately  compound  leaves  with  the  pinnate 
veining  in  simple  leaves,  such  as  elm.  Compare  a  palmately- 
veined  simple  leaf,  as  that  of  maple,  with  a  palmately  com- 
pound leaf  with  five  leaflets.  Such  comparisons  suggest 
that  the  leaflets  of  compound  leaves  correspond  to  the  lobes 
or  divisions  of  simple  leaves,  of  which  there  are  all  gradations 
from  small  notches  as  in  elm  leaf  and  lobes  as  in  oak  leaves 
to  such  extreme  division  as  in  the  celandine,  French  mari- 
gold, and  ragweed.  These  latter  are  almost  compound,  but 
show  narrow  strips  of  the  blade  connecting  the  divisions. 

Obviously,  it  is  often  difficult  to  distinguish  between  simple 
and  compound  leaves,  for  there  are  many  simple  leaves  so 
deeply  divided  as  to  resemble  closely  compound  leaves. 
The  only  reason  for  attempting  to  distinguish  in  such  cases 
is  for  convenience  in  describing  and  identifying  plants.  So 
far  as  functions  are  concerned,  the  compound  leaves  and 
the  deeply  divided  simple  leaves  are  apparently  equally 
efficient  in  permitting  light  to  reach  lower  leaves.  Observe 
such  plants  as  carrots  and  ragweed  in  bright  sunshine,  and 
note  how  the  light  filters  through  the  clefts  and  reaches 
lower  leaves.  Moreover,  since  breezes  frequently  sway  the 
leaves,  and  the  position  with  reference  to  light  constantly 
changes  with  that  of  the  sun,  the  result  is  that  lower  leaves 
are  quite  sure  to  get  more  or  less  direct  illumination  each 
day.  Undivided  leaves  which  exposed  the  same  surface 
on  the  same  branch  arrangement  would  certainly  keep  many 
of  the  lower  leaves  in  shade  most  of  the  time. 


196  APPLIED  BIOLOGY 


FLOWERS  OF  SEED-PLANTS 

189.  The  Functions  of  Flowers.  —  Our  studies  of  the  bean 
flower  (§  75)  may  now  be  applied  to  flowers  in  general.  The 
petals  and  sepals  of  flowers  serve  the  purpose  of  inclosing 
and  protecting  the  stamens  and  pistil,  which  are  the  parts 
of  flowers  essential  to  development  of  seeds.  The  important 
parts  of  the  stamens  are  the  anthers  or  pollen-cases  which 
contain  pollen-grains.  There  may  be  one  or  more  pistils. 
If  we  open  the  ovary  of  the  pistil  in  a  common  flower,  we 
see  one  or  more  rounded  bodies,  called  ovules;  and  by 
comparing  flowers  of  different  ages  it  is  made  evident  that 
from  each  ovule  a  seed  develops.  Inside  of  each  ovule  is 
an  egg-cell,  the  part  from  which  the  embryo  of  a  new  plant  will 
develop.  The  ovules  are  usually  easily  seen  by  the  unaided 
eye,  but  egg-cells  are  microscopic.  An  ovary  in  some  species 
contains  a  single  ovule,  with  a  single  egg-cell;  and  hence 
a  flower  with  one  such  ovary  could  form  only  one  seed  and 
one  plant.  Examples  are  buckwheat  and  "  four-o'clock." 
In  most  flowers  each  ovary  contains  more  than  one  ovule, 
and  hence  could  produce  as  many  seeds  as  there  are  ovules. 
Examples  are  the  bean  pod,  which  grows  from  a  single 
flower,  and  has  six  to  ten  seeds,  and  many  flowers  (e.g., 
poppy)  with  numerous  seeds  from  one  flower. 

The  pollen-grains  form  cells  which  are  often  called  male  re- 
productive cells  or  fertilizing  cells,  and  the  egg-cells  are  called 
female.  Neither  kind  of  cell  alone  is  able,  as  a  rule,  to  pro- 
duce a  seed ;  and  usually  there  must  first  be  a  union  of  the 
two  kinds  of  cells  as  a  preliminary  to  development  of  seeds 
(see  §  75,  on  bean  flower) .  There  are  a  few  cases  of  flowering 
plants  in  which  the  egg-cells  have  the  peculiar  power  of 
developing  without  the  aid  of  the  pollen-grains.  This  con- 
dition is  parthenogenesis. 

Pollination.  —  The  fertilizing  cells  produced  by  pollen- 
grains  must  be  brought  near  the  egg-cells'  in  the  ovary. 


STUDIES   OF  SEED-PLANTS 


197 


The  first  step  in  this  transfer  is  the  placing  of  the  grains  upon 
the  stigma  or  upper  end  of  the  pistil.  This  transferring  of 
pollen-grains,  called  pollination,  is  a  purely  mechanical 
process,  sometimes  done  by  insects,  sometimes  by  wind, 
sometimes  the  pollen  falls  from  anthers  to  stigma,  and  often 
gardeners  make  the  transfer  by  means  of  a  small  brush 
which  is  first  touched  to  anthers  and  then  to  stigmas  of 
flowers.  A  large  number  of  flowers  have  their  sepals,  petals, 
anthers,  stamens,  and  pistils  modified  to  fit  them  for  the 
visits  of  insects,  or  to  provide  for  pol- 
lination by  other  means;  and  such 
modifications  will  be  considered  later. 

Fertilization.  —  When  pollination  has 
occurred,  that  is,  when  pollen-grains 
have  lodged  on  the  stigma  of  the  pistil, 
the  wall  of  the  pollen-grain  grows  out 
into  a  tube  (pollen-tube)  and  this  grows 
down  between  the  cells  composing  the 
style  and  ovary  until  it  reaches  the 
egg-cell  in  the  ovule  (see  Fig.  61).  One 
pollen-cell  is  required  for  each  egg-cell, 
so  that  in  a  flower  with  a  dozen  egg- 
cells  there  must  be  a  dozen  pollen-tubes 
from  as  many  pollen-grains  growing 
down  the  style.  Many  more  pollen- 
grains  may  be  on  the  stigma  and  each 
may  grow  a  pollen-tube,  but  only  one 
is  needed  for  each  egg-cell. 

When  the  pollen-tube  has  nearly 
reached  the  egg-cell,  as  shown  in  Fig.  61, 
the  end  of  the  tube  opens  and  some  of  the  contained  proto- 
plasm and  a  nucleus  from  the  pollen  grain  slips  down  the 
tube  and  goes  into  the  egg-cell.  This  mass  which  enters  the 
egg-cell  is  the  fertilizing  cell  or  sperm-cell,  and  its  nucleus  is 
the  sperm-nucleus.  The  sperm-nucleus  soon  unites  with  the 


FIG.  61.  Longitudinal 
section  of  ovule  of 
pine,  with  two  egg- 
cells  (n,  o) ,  each  with 
a  nucleus,  p,  pollen- 
grains  ;  t,  pollen-tube ; 
e,  endosperm  ;  i,  in- 
tegument. The  en- 
tire ovule  forms  a 
seed  with  one  embryo, 
one  of  the  egg-cells 
not  developing.  (From 
Strasburger.) 


198 


APPLIED  BIOLOGY 


egg-nucleus,  and  the  combined  nucleus  is  the  nucleus  of  the 
fertilized  egg-cell.  This  process  of  fusion  of  sperm-cell  with 
egg-cell  is  fertilization.  Distinguish  be- 
tween pollination  and  fertilization  as 
defined  above. 

Cell-division.  —  The  result  of  fertiliza- 
tion is  to  stimulate  the  egg-cell  to  begin 
a  series  of  cell-divisions  to  form  a  group 
of  many  cells  (Fig.  62,  A,  C).  These 
cells  become  the  embryo,  which  is  the 
part  of  the  seed  able  to  develop  into 
a  new  plant.  While  the  egg-cell  is  de- 
veloping into  the  embryo,  the  surround- 
ing cells  of  the  ovule  are  forming  other 
parts  of  the  seed.  In  all  seeds  there  is 
the  outer  covering  or  seed-coat  of  one 
or  more  layers ;  and  in  many  seeds  there 
is  more  or  less  tissue  lying  between  the 
seed-coat  and  the  embryo.  This  is  com- 
posed of  cells  stored  with  food  to  be 
used  later  when  the  seed  is  sprouting  or 
germinating.  The  bean  seed  is  an  ex- 
ample of  one  with  an  embryo  and  seed- 
coat  only. 

A  castor-oil  bean  (§  138)  is  one  with 
an  embryo,  a  seed-coat,  and  a  large  mass 
of  food-storing  tissue  between  embryo 
and  seed-coat.  Such  food  tissue  outside  of  the  embryo  is 
called  endosperm  (meaning  within  the  seed). 

In  flowers  like  four-o'clock  and  buckwheat,  which  have  one 
ovule  and  form  one  embryo,  the  development  of  the  embryo 
and  its  surrounding  parts  (endosperm  and  seed-coats)  is 
associated  with  enlargement  and  change  in  the  wall  of  ovary. 
This  forms  an  outside  case  close  to  the  seed  so  that  the  fully- 
developed  seed  is  inclosed  in  a  sort  of  double  seed-coat. 


FIG.  62.  Stages  in  de- 
velopment of  a  seed- 
plant  embryo.  The 
four  upper  cells  in  A 
divide  many  times 
(B,  C)  and  form  the 
embryo  shown  in  D 
with  two  cotyledons 
(c),  epicotyl  (p),and 
hypocotyl  (K).  The 
row  of  cells  shown  be- 
low the  embryo  in 
each  figure  holds  it  in 
position  in  the  ovary. 
(From  Strasburger.) 


STUDIES  OF  SEED-PLANTS  199 

In  the  four-o'clock  seed  the  hard  outer  wall  has  developed 
from  the  wall  of  the  ovary,  the  inner  coat  is  the  real  seed-coat. 
In  the  buckwheat  the  "  hull,"  which  is  commonly  removed 
before  the  seed  is  marketed,  is  formed  from  the  wall  of  the 
ovary.  In  the  bean  flower,  which  is  an  example  of  an  ovary 
with  many  ovules,  a  number  of  egg-cells  are  usually  fertilized 
and  begin  development  at  about  the  same  time.  While  the 
ovules  are  developing  into  seeds,  the  wall  of  the  bean  ovary 
elongates  and  becomes  the  pod. 

In  all  flowering  plants  the  fully-developed  ovary  with  any 
part  of  the  flower  which  develops  with  it  is  called  a  fruit(§  212). 

190.  Special   Adaptations   of   Flowers.  —  Study   of   such 
simple    flowers    as    those    of    the   lily   family    and    of    the 
bean  plant  have  shown  the  essential  organs  and  work  of 
flowers    as    reproductive   structures.     In    order    to    adapt 
flowers  of  plants  under  special  conditions  to  the  all-important 
work  of  reproduction,  there  have  been  developed  numerous 
modifications ;  and  to  some  of  the  most  interesting  of  these 
special  adaptations  we  shall  now  give  some  attention.     Most 
important  of  the  adaptations  are  those  which  in  some  way  are 
related  to  securing  proper  pollination. 

191.  Flowers  and  Insects.  —  Many  of  the  most  wonderful 
adaptations  of  flowers  are  in  relation  to  insects.     There  is  a 
mutual  advantage  in  this  relation.     The  insect  secures  food 
(pollen  and  nectar).     Some  insects  like  moths  and  butter- 
flies, which  have  a  long  "  tongue,"  visit  flowers  for  the  nectar ; 
others  like  the  bees  and  wasps  collect  both  nectar  and  pollen. 
On  the  plant  side,  the  advantage  arises  from  the  fact  that 
while  getting  food  the  insects  brush  pollen  from  anthers  upon 
stigmas,    thus    producing    pollination   and  very  frequently 
cross-pollination  (placing  pollen  of  one  flower  on  stigma  of 
another  flower  of  the  same  species). 

192.  Advantage  of  Cross-pollination.  —  Many  experiments 
have  proved  that  plants  from  seeds  which  originated  through 
cross-pollination  are  usually  better  than  those  resulting  from 


200  APPLIED  BIOLOGY 

self-pollination  (pollen  on  stigma  of  its  own  flower).  Also 
in  some  cases  flowers  are  not  fertile  with  their  own  pollen, 
or  at  least  not  so  fertile  as  when  cross-pollinated.  Why 
these  things  are  so  is  unexplained  by  biology;  but  it  is  a 
well-established  fact  that  in  all  groups  of  animals  and  plants 
there  are  special  arrangements  for  producing  new  individuals 
from  egg-cells  fertilized  by  unrelated  sperm-cells  or  pollen- 
grains.  The  remarkable  contrivances,  described  in  the 
following  pages,  some  to  aid  cross-pollination  and  some  to 
prevent  self-pollination,  can  only  be  interpreted  as  meaning 
that  for  some  unknown  reason  it  is  important  that  plant 
ovules  should  be  fertilized  by  pollen-grains  from  other  flowers 
and  very  commonly  from  flowers  on  other  plants. 

However,  it  must  not  be  understood  that  self-pollination  is 
always  guarded  against,  for  there  are  some  flowers  which 
do  not  open  and  which  must  be  self-pollinated,  and  yet  they 
produce  good  seeds.  There  are  some  cases  of  flowers  which 
do  not  open,  but  which  may  possibly  be  pollinated  by  insects 
which  bite  holes  in  searching  for  food. 

193.  Adaptations  against  Self-pollination.  —  Most  famil- 
iar of  these  is  the  fact  that  in  many  species  certain  flowers 
have  only  stamens  and  others  have  only  pistils.  In  the  case 
of  cultivated  strawberry  plants,  some  varieties  produce 
only  pistils  in  their  flowers  and  are  called  pistillate  flowers. 
Other  varieties  have  both  stamens  and  pistils  in  each  flower 
and  are  said  to  be  perfect  or  bi-sexual  flowers.  There  are  no 
varieties  of  strawberries  which  have  only  stamens  (staminate 
flowers) ;  but  some  other  kinds  of  plants  have  such  flowers. 
Strawberry  plants  which  are  known  to  produce  only  pistillate 
flowers  must  be  planted  near  plants  which  have  perfect 
flowers.  In  buying  plants  from  dealers  one  must  learn  the 
name  of  the  variety  and  then  find  out  from  catalogues  or 
garden  books  whether  the  variety  is  perfect  or  pistillate. 
A  very  instructive  experiment  consists  in  planting  some 
strawberry  plants  of  a  pistillate  variety  in  several  pots  or 


STUDIES  OF  SEED-PLANTS  201 

boxes,  keeping  covered  with  mosquito-netting  to  ward  off 
insects,  and  then  pollinating  the  flowers  of  the  plants  in  one 
or  more  pots,  using  a  small  brush  or  a  feather  to  transfer 
the  pollen  from  stamens  of  the  perfect  to  the  pistillate  flowers. 
Perfect  and  pistillate  plants  for  such  an  experiment  can  be 
purchased  from  seed-dealers  for  about  30  cents  per  dozen, 
postpaid,  in  autumn  or  spring. 

A  second  common  adaptation  against  self-pollination  is 
found  in  some  flowers  in  which  the  pollen  and  the  stigma 
of  a  flower  are  not  mature  (ready  for  pollination)  at  the  same 
time,  but  since  the  flowers  open  at  different  times  it  is  possible 
for  insects  to  produce  cross-pollination. 

Still  another  kind  of  protection  against  self-pollination  is 
illustrated  by  some  flowers  which  have  stamens  much  shorter 
than  the  style  so  that  pollen  cannot  fall  upon  the  stigma  of 
the  same  flower,  making  pollination  by  insects  necessary. 
Also,  there  are  other  flowers  (e.g.,  the  bean  and  orchids)  which 
have  their  corollas  peculiarly  modified  so  that  at  the  same 
time  they  guard  against  self-pollination  and  favor  cross- 
pollination. 

194.  Adaptations  for  Cross-pollination.  —  The  simplest  of 
the  arrangements  for  cross-pollination  is  connected  with 
wind  as  an  agency  for  carrying  pollen.  Great  quantities 
of  light  pollen  are  produced  by  many  plants,  and  this  floats 
upon  currents  of  air  to  the  stigmas  of  other  flowers.  Indian 
corn  and  pines  are  good  examples.  Many  thousand  species 
of  plants  are  known  to  be  cross-pollinated  by  the  wind. 

Some  of  the  adaptations  for  cross-pollination  by  insects 
are  among  the  most  remarkable  structures  in  the  plant 
kingdom.  The  bean  flower  as  an  example  has  been  described 
in  §  75.  Probably  most  flowers  which,  like  that  of  bean 
plant,  are  irregular  in  shape  (not  radially  symmetrical) 
are  pollinated  by  insects.  In  some  of  the  orchids  (e.g., 
lady-slipper)  the  flower  has  a  peculiar  sac  surrounding  the 
anthers  and  the  stigma,  and  through  this  insects  must  crawl 


202  APPLIED  BIOLOGY 

in  order  to  reach  nectar.  Pollen  rubbed  on  the  insect's  body 
while  in  one  flower  will  be  brushed  off  when  crawling  past 
the  stigma  of  another  flower.  Many  such  peculiar  arrange- 
ments are  described  in  Charles  Darwin's  great  work  on  "  Cross- 
and  Self-fertilization  in  the  Vegetable  Kingdom." 

195.  Structure  of  Various  Flowers.  —  (L)  In  addition  to  some 
simple  flower,  and  the  bean  flower  previously  studied  (§  74),  the 
student  should  study  a  number  of  other  types.  If  this  work  is  left 
for  the  spring  of  the  year,  a  few  collecting  excursions  will  furnish  an 
abundance  of  materials  for  study  (trillium,  buttercup,  mustard,  apple 
or  pear  or  cherry,  dandelion,  violet,  anemone,  hepatica,  adder's  tongue, 
dogwood,  spring-beauty).  Any  such  flowers  may  be  collected 
in  the  proper  season  and  preserved  in  a  solution  of  five  parts 
commercial  formalin  in  one  hundred  of  water,  in  fruit-jars.  In 
winter  there  are  scillas,  tulips,  hyacinths  (especially  white  Roman), 
snowdrop,  narcissus,  freesias,  fuchsias,  and  primroses  available  in 
greenhouses,  some  of  them  at  small  cost,  especially  if  arrange- 
ments are  made  with  gardeners  for  growing  ordinary  varieties. 
Plants  with  bulbs  can  easily  be  brought  into  flower  in  schools  by 
planting  the  bulbs  in  pots  or  boxes  in  early  autumn,  leaving  in  a  cold 
cellar  or  buried  outdoors  for  two  months,  and  then  bringing  into 
warm  and  light  rooms.  Or  if  planted  in  gardens  according  to  direc- 
tions in  dealers'  catalogues,  they  will  bloom  in  very  early  spring. 

The  time  available  for  laboratory  study  of  flowers  should  be  used 
in  careful  examination  of  as  many  kinds  as  possible,  comparing  with 
flowers  previously  studied,  and  making  brief  notes  and  simple 
sketches.  Only  a  small  proportion  of  the  time  should  be  used  for 
making  notes  and  sketches ;  it  is  better  to  understand  the  parts  of 
a  dozen  types  of  flowers  than  to  have  made  a  beautiful  drawing 
in  detail  and  have  studied  only  one  flower. 

It  will  be  well  to  read  the  following  pages  in  advance  of  the  labora- 
tory study,  and  then  later  study  the  text  carefully  in  the  light  of  the 
facts  learned  by  examination  of  flowers  themselves. 

It  is  recommended  that,  beginning  with  the  first  flower  studied, 
a  record  be  made  in  the  note-book  of  answers  to  the  following 
questions :  — 

Are  sepals,  petals,  stamens,  and  pistil  present  in  flowers  examined, 
and  how  many  of  each  ?  Record  your  observations  in  tabular  form 
under  headings  such  as  the  following  :  — 

Name  of  flower        Sepals        Petals        Stamens        Pistil 


STUDIES   OF  SEED-PLANTS  203 

196.  Petals  and  Sepals.  —  The  simplest  flowers  have  only 
the  essential  reproductive  parts,  the  stamens  and  pistils, 
the  petals  and  sepals  being  absent.     Such  flowers  may  have 
both  stamens  and  pistils  (i.e.,  are  perfect) ;   or  some  flowers 
may  have  stamens  (staminate),  and  others  pistils  (pistillate). 
Willows  are  familiar  examples  of  the  latter. 

In  many  flowers,  particularly  of  the  lily  family,  sepals  and 
petals  are  present,  but  they  are  all  alike  in  size  and  color. 
In  such  cases  where  we  cannot  distinguish  between  calyx 
and  corolla,  the  term  perianth  is  often  used.  In  the  most 
common  flowers  the  petals  are  larger  than  the  sepals,  and 
usually  white  or  colored.  The  sepals  in  some  flowers  are 
small,  scale-like  structures,  and  in  others  they  may  be  leaf- 
like.  Some  flowers  (e.g.,  anemone,  hepatica,  rue  anemone, 
marsh  marigold)  have  no  petals,  but  their  sepals  resemble 
petals.  Some  flowers  of  this  kind  are  at  first  puzzling, 
because  the  leaf-like  bracts  (collectively  called  involucre) 
on  the  flower-stalk  may  be  so  close  to  the  flowers  that  they 
are  at  first  taken  to  be  green  sepals.  Hepatica  is  such  a 
puzzle,  but  in  the  closely  related  anemone  flower  the  bracts  are 
not  near  enough  to  be  mistaken  for  sepals.  In  the  dogwood 
flower  (really  a  cluster  of  small  flowers)  the  bracts  are  large 
and  petal-like,  and  are  commonly  mistaken  for  petals  by 
those  who  have  not  studied  botany. 

Modification  of  Petals.  —  In  the  simplest  flowers  the 
petals  are  free  from  one  another;  but  in  the  highest  seed- 
plants  they  are  more  or  less  united  into  bell-shaped  or  tubular 
corollas.  The  sepals  may  also  be  united,  as  in  the  tobacco 
flower ;  or  they  may  be  separate  while  the  petals  are  united 
so  completely  that  only  the  lines  of  fusion  indicate  the  number 
of  united  petals  (as  in  the  morning  glory).  Between  these 
two  extremes  of  fusion  of  petals  and  sepals  there  are  numerous 
intermediate  stages  which  may  be  found  in  common  flowers. 

197.  Number  of  Parts  of  Flowers.  —  In  some  flowers  the 
number  of  sepals,  petals,  and  stamens  is  the  same  (often  three, 


204  APPLIED  BIOLOGY 

four,  or  five) ;  but  many  other  flowers  vary  as  to  the  number 
of  these  parts. 

There  is  no  essential  relation  between  number  of  parts 
and  the  reproductive  function  of  flowers.  Three  sepals  in 
one  flower  may  inclose  the  flower  bud  as  well  as  five  or 
more  do  in  some  other  flower ;  a  few  petals  which  are  large 
and  conspicuous  may  be  more  useful  in  relation  to  insects 
than  are  many  small  petals ;  a  few  stamens  may  be  large  and 
produce  more  pollen  than  numerous  stamens;  and  there  is 


sr  e 

FIG.  63.     Relation  of  the  ovary  and  receptacle   in  flowers,     r,  receptacle ; 
o,  ovary.      (From  Strasburger.) 

no  necessary  relation  between  number  of  styles  and  stigmas 
and  the  number  of  seeds  which  may  be  produced. 

It  is  interesting  to  note  that  many  monocotyledon  (§  141) 
flowers  have  their  parts  in  threes,  and  many  dicotyledons 
have  their  parts  in  fives  or  fours.  References  to  such  con- 
stant numbers  in  certain  families  of  plants  will  be  found  in 
books  for  identification. 

198.  Position  of  Ovary  in  Flowers.  —  In  very  many  flowers 
the  sepals,  petals,  and  stamens  appear  attached  beneath 
the  ovary  (Fig.  63,  A ) .  In  some  cases  (e.g. ,  apple,  squash) ,  the 
ovary  appears  to  be  entirely  beneath  the  flower;  that  is, 
the  sepals,  petals,  and  stamens  appear  attached  above  the 
ovary  (Fig.  63,  C).  There  are  still  other  flowers  which  are  in- 
termediate, as  shown  in  B  and  B'  of  the  same  figure.  Special 
names  are  often  applied  to  these  conditions  when  describing 
species. 


STUDIES  OF  SEED-PLANTS 


205 


B  C 

FIG.  64.  Compound  pistils. 
A,  two  pistil-leaves  or  car- 
pels slightly  united;  B, 
three  united  as  to  ovaries, 
but  with  separate  styles  ; 
C,  three  united  into  one 
compound  ovary  and 
style.  (From  Gray.) 


199.  Simple   and    Compound   Pistils.  —  In  many  flowers 
there  is  a  single  pistil,  which  appears  to  be  formed  from  a 
single  leaf  -like  structure  known  as  a 

carpel.     If  we  imagine  a  small  nar- 

row leaf  with  one  or  more  ovules 

attached  on  its  margin,  and  then  this 

leaf  rolled  so  as  to  bring  its  edges  to- 

gether and  the  ovules  inside  the  in- 

closed  cavity,    the   formation   of  a 

simple  pistil  from  a  carpel  would  be 

illustrated. 

In    buttercups    there    are    many 

simple  pistils  in  a  flower.     In  very 

many  flowers  the  pistils  have  united 

together  to  form  a  compound  pistil. 

In  some  of  these  the  ovaries  of  the 

pistils  are  more  or  less  united,  but 

there  are  as  many  styles  as  there  were  pistils  (Fig.  64,  A  and 

B).     In  others  there  is  complete  union  so  that  there  is  one 

ovary  and  one  style  (Fig.  64,  (7),  and  only  careful  examina- 
tion of  grooves  on  the  surface  and  of 
sections  shows  that  the  pistil  is  com- 
pounded of  two  or  more  simple  pistils. 
Fig.  65,  A,  B,  shows  in  cross  section 
two  kinds  of  ovaries  formed  from 
three  carpels.  In  A  the  carpels  ap- 
pear  to  have  united  at  the  edges, 
Caving  a  single  cavity  in  the  ovary 
and  the  ovules  attached  to  the  outer 
walj  .  while  in  £  the  arrangement 

.        .    °    .       . 

suggests  that  three  simple  pistils 
have  united  completely,  so  that  the  ovules  are  attached  to 
a  central  column. 

200.  Irregular  Flowers.      In   the    bean    flower   we   have 
already  studied  a  good  example  of  irregular  flowers;  that  is, 


FIG.  65.      Transverse    sec- 
tions   of     two      ovaries, 


with  one  chamber  and  in 
B  with  three  chambers. 

w,  ovary  wall. 


206  APPLIED  BIOLOGY 

flowers  which  are  not  radially  symmetrical,  as  are  those  of 
apple,  buttercup,  tulip,  and  many  others  equally  well  known. 
In  the  irregular  flowers  of  bean,  sweet  pea,  violets,  pansy, 
salvia  and  many  others  the  petals  are  separate;  but  there 
are  many  irregular  flowers  with  united  petals.  Flowers  of 
toad-flax,  snapdragon,  mullein,  catalpa,  and  any  mint  are 
good  examples.  In  most  cases  irregular  flowers  appear  to 
be  adapted  to  insect  visits.  This  does  not  mean  that  regular 
flowers  became  irregular  because  insects  began  to  visit  them, 
nor  in  order  to  allow  or  to  encourage  insects  to  make  visits ; 
but  rather  that  flowers  which  were  irregular  were  thereby 
fitted  to  insect  visits.  Just  what  caused  flowers  to  become 
irregular  in  the  first  place  is  unknown. 

201.  Double  Flowers.  —  A  large  number  of  varieties  of 
cultivated  flowers  have  become  "  double,"  which  means 
that  the  petals  have  become  very  numerous  and  the  stamens 
and  pistils  have  largely  or  entirely  disappeared.  In  some 
varieties  (e.g.,  the  double-flowered  portulacas)  some  flowers 
do  not  develop  completely  "  double,"  but  have  pistils  and 
stamens  which  produce  seeds.  When  these  seeds  develop 
into  new  plants,  some  of  the  flowers  produced  will  be 
"  double  "  and  others  only  partly  so.  It  is  impossible  to 
continue  to  raise  only  completely  double  flowers  from  seed, 
for  they  would  be  seedless.  If  a  gardener  wants  a  bed  of  such 
flowers,  he  must  propagate  entirely  from  cuttings.  In  the 
case  of  many  varieties  of  roses  this  propagation  from  cuttings 
is  the  exclusive  method  used  by  gardeners,  and  we  never 
see  stamens  and  pistils  in  flowers  of  these  varieties. 

The  explanation  of  double  flowers  is  often  said  to  be  that 
the  stamens  and  pistils  have  changed  to  petals.  One  fact 
which  at  first  sight  seems  to  support  this  view  is  that  in 
some  flowers  (e.g.,  water-lily)  there  are  all  stages  of  transition 
between  petals  and  stamens.  However,  this  does  not  prove 
that  stamens  and  pistils  were  originally  petals.  On  the  con- 
trary, stamens  and  pistils  appeared  in  the  lowest  flowers 


STUDIES   OF  SEED-PLANTS  207 

before  petals  did.  Moreover,  the  fact  that  sometimes  green 
petals  are  found  does  not  prove  that  petals  were  once  green 
leaves.  Also,  colored  sepals  and  bracts  were  not  once  petals. 
But  the  fact  that  sepals,  petals,  stamens,  and  pistils  all  suggest 
more  or  less  similarity  to  leaves  probably  means  that  they 
have  developed  or  been  evolved  from  expanded  leaf-like 
structures  in  lower  plants  and  that  foliage  leaves  have  de- 
veloped from  similar  structures. 

202.  Flower     Regarded     as     Shortened     and     Modified 
Branch.  —  Every  flower  begins  as  a  set  of  leaf-like  structures 
arranged  in  whorls  or  circles  on  the  receptacle,  which  is 
simply  the  conical  end  of  the  flower-stalk.     As  the  sepals, 
petals,  and  stamens  grow  up,  the  receptacle  may  remain 
conical  (as  in  Fig.  63,  A),  or  the  receptacle  may  become  more 
or  less  cup-shaped,  resulting  in  flowers  like  B  and  Bf  in  Fig.  63. 
And  if  the  receptacle  both  grows  cup-shaped  and  at  the  same 
time  closely  surrounds  the  ovary,  the  result  is  a  flower  like 
Fig.  63,  C,  in  which  the  ovary  is  beneath  the  sepals  and  petals. 

203.  Stamens  are  usually  entirely  free  from  each  other, 
but  in  some  flowers  they  are  united  either  by  the  anthers  or 
by  the  filaments  into  a  ring,  and  in  some  flowers  into  two  or 
three  groups  or  more.     In  the  bean  flower  there  are  two  groups, 
with  nine  stamens  in  one,  and  one  in  the  other.     In  the  dande- 
lion (§  207)  the  stamens  are  united  into  a  ring,  which  sur- 
rounds the  pistil. 

The  anther  is  the  essential  part  of  the  stamen.  In  order 
to  get  acquainted  with  some  of  the  various  forms  of  anthers, 
they  should  be  carefully  examined  in  the  flowers  studied. 
Use  a  hand-lens,  and  also  remove  some  pollen  for  examina- 
tion with  the  compound  microscope. 

Anthers  are  in  some  flowers  attached  directly  and  without 
filaments.  They  are  then  said  to  be  sessile,  just  as  a  leaf 
attached  without  a  petiole  is  sessile. 

(D)  Germination  of  pollen-grains  and  growth  of  pollen-tubes  may 
be  studied  by  placing  some  pollen,  if  possible  from  several  kinds  of 


208  APPLIED  BIOLOGY 

plants,  in  concaved  object-slides  containing  5,  10  or  15  per  cent  of 
white  sugar  in  water  (sugar-solution,  or  syrup).  The  slides  should 
be  kept  covered  to  prevent  evaporation.  Examine  from  time  to  time 
with  the  low  power  of  the  microscope.  It  is  well  to  use  several  kinds 
of  pollen  and  various  strengths  of  syrup,  for  the  grains  do  not  all 
germinate  in  the  same  strength  of  sugar-solution.  Sometimes  it  is 
possible  to  find  pollen-grains  beginning  to  germinate  on  the  sticky 
stigmas  of  flowers. 

204.  Introductory    Study    of    Flowers.  —  The    foregoing 
account  is  intended  to  be  merely  an  introduction  to  the  study 
of  flowers.     Their  forms  are  so  numerous  that  only  brief 
interpretation  of  some  of  the  most  common  modifications 
can  be  attempted  in   a  year's  course  in  biology;  and  so 
the  authors  have  selected  for  study    the  materials  which 
are  most   likely  to   be   interesting   and   useful  to  general 
readers. 

Those  who  are  especially  interested  in  the  forms  and  adap- 
tations of  flowers  should  read  the  accounts  in  the  standard 
textbooks  of  botany  by  Coulter,  Bergen,  Atkinson,  Bailey, 
Gray ;  and  then  perhaps  such  famous  books  as  Darwin's 
"Cross-  and  Self-fertilization  in  the  Vegetable  Kingdom,"  and 
the  Chapters  on  flowers  in  Kerner's  "  Natural  History  of 
Plants." 

205.  Flower-clusters :     Inflorescence.  —  In    many    small 
plants  there  is  only  one  flower,  and  in  others  the  few  flowers 
appear  singly  in  the  axils  of  ordinary  foliage  leaves.     But 
in  the  great  majority  of  the  common  seed-plants  the  tend- 
ency is  to  produce  a  number  of  flowers  near  together  in 
groups  or  clusters.     The  term  inflorescence  refers  to  such 
flower-clusters.     Fig.  66,  A-G  illustrates  some  of  the  most 
common  arrangements.     The  main  lines  in  each  of  these 
figures  represent  the  main  flower-stalk  (peduncle),  and  the 
branch  lines  show  the  position  of  the  individual  flower-stalks 
(pedicels).     If  time  allows  special  study  of  flower-clusters, 
the  textbooks  of  botany  by  Gray,  Bergen,  or  others  should  be 
consulted. 


STUDIES   OF  SEED-PLANTS 


209 


206.  The  Meaning  of  Flower-clusters.  —  The  value  of 
flower-clusters  is  believed  to  be  threefold;  namely,  (1)  to 
attract  insects  which  carry  pollen,  (2)  to  expose  certain  flowers 
for  pollination  by  wind,  (3)  to  aid  in  distributionof  seeds. 


FIG.  66.  Types  of  flower-clusters.  The  positions  of  individual  flowers  are 
represented  by  the  black  spots.  The  order  of  flowering  is  shown  by  the 
numbers.  A,  raceme  ;  B,  spike  ;  C,  simple  umbel  ;  D,  head  ;  E,  corymb  ; 
F,  compound  umbel ;  G,  cyme.  Note  that  the  first-formed  flower  is  in 
center  of  G,  but  at  edge  of  clusters  shown  in  C,  D,  E,  F. 

With  regard  to  insects,  a  cluster  of  small  flowers  held  above 
the  leaves  of  the  plant  by  the  flower-stem  appears  to  be  in 
a  particularly  advantageous  position  for  attracting  attention. 
Certainly  they  are  made  more  conspicuous  to  the  human  eye, 
but  the  truth  is  that  it  is  not  yet  certain  whether  most  in- 
sects which  visit  flowers  are  guided  by  the  brilliant  colors 
of  the  flowers.  It  is  well  known  that  many  inconspicuous 
flowers  are  favorites  of  insects,  which  are  probably  led  to  the 
flowers  by  odors  more  than  by  colors.  However,  many 
botanists  believe  that  conspicuous  flowers  do  aid  in  attracting 


210  APPLIED  BIOLOGY 

insects ;  and  if  this  is  true,  grouping  of  flowers  in  clusters  is 
surely  important. 

Concerning  the  suggestion  above  that  flower-clusters  favor 
pollination  by  the  wind,  it  is  evident  that  flowers  held  above 
the  leaves  of  a  plant  have  a  most  advantageous  position  both 
for  distributing  their  pollen  to  the  wind  and  for  receiving 
it  from  some  other  plant.  Notice  a  field  of  grass,  oats,  wheat, 
or  corn  when  in  flower,  and  observe  how  well  the  flower- 
clusters  are  exposed  to  the  wind.  Imagine  these  flowers 
scattered  along  the  stem  where  the  main  leaves  are,  and  it 
at  once  becomes  clear  that  the  wind  could  not  scatter  the 
pollen  widely. 

Finally,  the  value  of  flower-clusters  for  seed-distribution 
is  based  on  the  same  facts  as  cited  above  for  pollination  by 
wind.  Any  one  who  has  watched  the  wind  scatter  the  seeds 
of  thistle,  dandelion,  or  trees  which  have  winged  seeds,  will 
understand  at  once  how  flower-clusters  in  their  usual  exposed 
positions  are  of  great  advantage  in  scattering  seeds. 

207.  Flowers  in  Heads.  —  What  are  commonly  called 
flowers  of  clover,  dandelion,  etc.,  are  not  single  flowers,  but 
clusters  of  flowers  set  on  the  expanded  end  of  a  flower-branch 
(Fig.  66,  D).  When  thus  set  in  heads  and  crowded  closely 
together,  the  constituent  flowers  are  very  small.  A  dande- 
lion flower-head  (commonly  called  dandelion  flower),  a  sun- 
flower, or  some  similar  flower-head  should  be  examined. 

Dandelion  Flower-head.  —  (L)  Dandelion  plants  taken  up  in  fall 
or  early  spring  are  easily  grown  in  pots  or  boxes  in  the  schoolroom, 
and  flowers  in  various  stages  of  development  obtained. 

Examine  the  peculiar  flower-stem  which  supports  the  head  of 
flowers.  As  the  flowers  grow  old  and  the  head  becomes  closed  by 
the  surrounding  bracts,  notice  whether  this  flower-stem  grows 
longer,  and  especially  notice  its  length  when  the  seeds  are  mature 
and  about  to  be  blown  away  by  the  wind. 

Notice  that  beneath  the  flower-head  are  two  rows  of  bracts, 
collectively  forming  an  involucre.  Observe  the  position  of  these 
bracts  in  young  heads,  and  in  old  ones  which  have  closed.  These 


STUDIES  OF  SEED-PLANTS 


211 


bracts  correspond  to  leaves  on  stems  beneath  ordinary  flowers,  and 
not  to  the  calyx.  The  white  petal-like  structures  surrounding 
clusters  of  dogwood  flowers  are  also  examples  of  bracts. 

Any  one  of  the  numerous  flowers  in  a  dandelion  flower-head  has 
the  structure  shown  in  Fig.  67,  a.  The  tuft  of  bristles  or  hairs  (called 
pappus)  and  the  ring  below  the  hairs  represent  the  calyx.  The  sur- 
rounding corolla  is  tubular  at  its  base  and  the  upper  part  is  a 
flattened  strap-like  structure 
called  a  ligule.  Five  petals 
are  united  in  this  peculiar 
corolla.  It  is  evident  then 
that  the  petal-like  structures 
which  first  attract  attention 
when  one  casually  looks  at 
a  dandelion  flower-head  are 
simply  the  strap-like  exten- 
sions from  as  many  corollas. 
The  five  stamens  are  united 
at  their  anthers,  and  protrud- 
ing up  through  the  center  of 
this  ring  of  stamens  is  the 
style,  which  is  forked.  The  JTIG  57 
position  of  the  ovary  is  the 
same  as  that  shown  at  the 
bottom  of  Fig.  67,  a,  c. 

Sunflower,  Aster,  Calen- 
dula, Coreopsis,  Zinnia, 
Marigold.  —  (L)  These  are 
common  flower-heads  with 
structure  similar  to  that  of  the  dandelion,  except  that  only  the  flowers 
at  the  margin  of  the  head  have  the  strap-like  rays.  All  the  other 
flowers  in  the  heads  have  the  same  structure;  they  are  tubular 
flowers  without  rays.  The  flowers  without  the  straps  are  called  disc- 
flowers,  those  with  straps  are  ray-flowers.  It  is  evident  that  the 
conspicuousness  of  sunflowers  and  their  relatives  named  above  is 
due  largely  to  the  large  and  colored  rays  of  the  marginal  flowers. 

Flower-heads  of  Common  Thistles,  Burdock,  Centaurea,  Tansy,  Iron- 
weed.  —  (D)  These  have  no  strap-like  rays  on  any  of  the  corollas 
in  the  head.  In  thistles  all  flowers  are  alike  and  similar  to  the 
disc-flowers  of  the  sunflower  head.  In  centaurea  the  marginal 
flowers  are  enlarged  and  without  stamens  and  pistils  (i.e.,  they 
are  sterile.) 


a      o  v       c 

Composite  flowers  of  Arnica,  a, 
ray-flower  ;  b,  disc-flower  consisting  of  a 
calyx  with  bristles,  a  tubular  corolla 
with  five  lobes,  style  divided  into  two 
stigmas,  stamens  united  in  ring  around 
the  style  ;  c,  longitudinal  section  of  b. 
Some  composites  have  stamens  in  ray- 
flowers  (a).  (From  Strasburger .) 


212  APPLIED  BIOLOGY 

The  dandelion,  sunflower,  aster  and  the  others  mentioned 
above  are  members  of  the  family  of  plants  known  in  botany 
as  the  Composite  Family  (Compositse).  It  is  the  largest 
family  of  seed-plants  (phanerogams).  There  are  over  five 
hundred  species  of  composites  in  the  United  States  east  of 
the  Mississippi  River.  The  composites  are  regarded  as  the 
highest  plants,  just  as  mammals  are  the  highest  animals. 

It  is  easy  to  identify  a  specimen  flower  as  a  composite,  but 
in  order  to  find  out  what  its  name  is,  one  must  consult  special 
books  on  the  classification  of  plants.  Most  famous  of  such 
books  is  the  "  Manual  of  Botany,"  by  Asa  Gray,  the  most 
noted  American  botanist,  professor  of  natural  history  at 
Harvard  College  from  1842-1888. 

208.  Flowers  and  Applied  Biology.  —  How  any  relation 
can  exist  between  study  of  flowers  and  applied  biology  may 
not  be  apparent  to  those  persons  whose  idea  of  "  applied  " 
is  that  it  means  something  good  to  eat  or  something  of 
economic  value,  as  are  forests,  cereal  plants,  etc.  And  yet 
it  would  be  a  mistake  to  think  of  flowers  as  not  having 
economic  value,  for  millions  of  dollars  are  invested  in  land 
and  greenhouses  used  solely  for  producing  flowers  for  the 
market,  and  in  all  civilized  lands  the  sale  of  flowers  amounts 
to  an  enormous  total  of  dollars  annually. 

In  still  another  way  study  of  flowers  is  of  direct  practical 
use,  namely,  in  producing  new  varieties  of  plants.  Each 
year  the  catalogues  of  dealers  announce  new  and  attractive 
varieties.  Many  of  these  have  been  produced  by  gardeners 
who  have  applied  their  knowledge  of  flowers  to  experiments 
with  artificial  cross-pollination;  and  the  result  of  such  ex- 
periments is  often  valuable  new  varieties  of  plants. 

But  while  some  readers  may  not  be  interested  in  flowers 
because  some  other  persons  make  great  profits  in  growing 
and  selling  them,  it  is  probable  that  there  are  few  individuals 
who  will  admit  that  they  have  no  interest  in  flowers  for  their 
ornamental  value.  Have  you  ever  met  a  person  so  dead 


STUDIES   OF  SEED-PLANTS  213 

to  all  appreciation  of  the  beautiful  that  he  did  not  some- 
times show  interest  in  flowers?  Perhaps  you  have;  but 
such  persons  are  becoming  rare.  Study  of  flowers  was  once 
thought  to  be  a  subject  for  girls,  but  in  recent  years  it  has 
become  more  and  more  evident  that  appreciation  of  the  beauty 
in  nature  is  just  as  valuable  for  men  as  for  women.  In  many 
colleges  for  men  there  are  now  courses  in  aesthetics,  the  science 
of  the  beautiful. 

It  is  evident,  then,  that  some  outline  study  of  flowers 
belongs  in  applied  biology;  for  besides  the  application  of 
such  knowledge  to  the  business  of  growing  plants  and  of 
producing  new  varieties  mentioned  above,  an  understanding 
of  the  nature  of  flowers  is  useful  to  every  cultured  citizen 
who  has  any  aesthetical  appreciation  whatever.  Flower 
study  is  certainly  applicable  to  our  daily  life,  if  we  mean  life 
in  the  widest  sense  of  the  word. 

These  comments  on  the  importance  of  appreciating  the 
beauty  of  natural  things  have  been  suggested  in  connection 
with  flowers,  because  to  most  people  flowers  are  the  most 
beautiful  and  remarkable  of  natural  objects.  However,  in 
many  other  structures  of  animals  and  plants  there  are  things 
which  appeal  to  our  sense  of  the  beautiful  (aesthetic  sense), 
and  the  student  of  biology  should  keep  constantly  on  the 
lookout  for  them. 

SEED-PLANTS   WITHOUT   TRUE   FLOWERS 

209.  Angiosperms  and  Gymnosperms.  —  So  far  the  de- 
scriptions of  plant  reproductive  organs  have  applied  to  the 
flowers  found  among  monocotyledons  or  dicotyledons.  All 
these  have  the  seeds  formed  inside  the  closed  ovaries  of 
flowers,  and  are  classified  in  a  great  group  called  Angio- 
sperms (meaning  seed-vessel,  i.e.,  ovary).  In  addition  to  such 
seed-plants  with  closed  ovary,  there  are  several  hundred 
species  of  plants  which  form  seeds,  but  which  have  the  seeds 
unprotected  by  an  ovary.  Such  plants  form  a  group  known 


214  APPLIED  BIOLOGY 

as  Gymnosperms  (meaning  naked  seeds).  The  most  familiar 
examples  are  the  conifers  (a  word  meaning  cone-bearers), 
including  pine,  hemlock,  larch,  spruce,  and  similar  trees. 
Other  gymnosperms  are  the  cycads  seen  in  greenhouses,  and 
the  Japanese  gingko  tree  in  American  parks. 

The  Angiosperms  and  Gymnosperms  together  constitute 
the  great  division  of  plants  known  as  Spermaphytes  (meaning 
seed-plants).  Seed-plants  are  by  some  botanists  called 
Phanerogams  (meaning  visible  organs  of  reproduction,  i.e., 
flowers).  The  term  seed-plants  is  better  than  flowering 
plants,  because  gymnosperms  do  not  have  true  flowers ;  and, 
strictly  speaking,  flowering  plants  would  include  only  an- 
giosperms.  The  following  table  shows  the  relations  of  these 
groups  of  plants  mentioned  above. 

Groups  of  Seed-plants 
Spermaphytes  Gymnosperms 

Phanerogams  Angiosperms  Monocotyledons 

Dicotyledons 

210.  Gymnosperm  Reproductive  Organs.  —  Only  these 
organs  of  gymnosperms  are  selected  for  special  mention,  be- 
cause in  stem,  roots,  and  leaves  there  is  great  similarity  to 
angiosperms  already  studied.  The  stems  of  the  conifers 
have  the  general  plan  of  the  dicotyledons,  with  secondary 
growth  causing  annual  enlargement  of  diameter  (Fig.  46). 
Most  conifers  have  needle-like  leaves  adapted  to  severe 
conditions  of  weather,  cycads  have  leaves  suggesting  those  of 
palms  (which  are  monocotyledons),  the  gingko  tree  has 
broad  leaves,  and  the  arbor-vitae  has  scale-like  leaves.  Also, 
most  of  the  conifers  are  evergreens,  that  is,  their  leaves  live 
several  years  and  do  not  all  fall  at  a  certain  season ;  but  the 
common  larch  or  tamarack  is  a  conifer  that  sheds  its  leaves 
annually  like  most  dicotyledon  trees,  and  there  are  many 
evergreen  shrubs  and  trees  among  angiosperms.  In  short, 
there  is  no  important  characteristic  of  leaves  in  gymnosperms 


STUDIES   OF  SEED-PLANTS 


215 


which  is  not  also  found  among  angiosperms.  In  the  seeds 
of  conifers  there  may  be  more  than  two  cotyledons  (from  two 
to  twelve) ;  but  botanists  do  not  attach  any  special  signifi- 
cance to  the  mere  number,  since  in  other  respects  conifer  and 
dicotyledonous  embryos  are  very  similar. 

It  is  then  in  the  reproductive  organs  that  there  is  the  most 
striking  difference  between  gymnosperms  and  angiosperms; 
and  hence  the  two  groups  are  compared  in  this  connection 
with  the  lesson  on  the  reproductive  organs  (flowers)  of  the 
highest  plants. 

211.  Cones  as  Reproductive  Organs.  —  The  pine  or  other 
common  cone-bearing  tree  may  be  taken  as  a  type  of  the 
reproduction  found  in  gymnosperms.  In  addition  to  the 
foliage  leaves,  certain  leaves  are 
set  apart  for  purposes  of  repro- 
duction. These  leaves  (called 
sporophylls,  or  spore-leaves)  are 
arranged  in  the  form  of  cones,  of 
which  there  are  two  kinds. 

One  kind  of  cone  ("male")  is 
small  (Fig.  68,  A),  and  each  spore- 
leaf,  corresponding  to  a  stamen  of 
flowers,  bears  on  its  lower  surface 
spore-cases  or  pollen-sacs,  which 
produce  pollen-grains.  Such  a 
cone  is  often  called  staminate, 
the  term  commonly  applied  to  flowers  which  have  only 
stamens. 

The  other  kind  of  cone  ("  female  ")  has  a  similar  structure, 
but  is  larger.  It  is  the  well-known  cone  often  seen  on  cone- 
bearing  trees  (Fig.  68,  B).  Each  spore-leaf  of  the  larger  cones 
has  on  its  upper  surface  spore-cases  or  ovules  (two  in  pine) ; 
and  such  a  spore-leaf  is  equivalent  to  a  carpel,  with  its  ovules, 
in  true  flowers  of  angiosperms.  In  true  flowers  the  carpel 
folds  and  grows  together  to  form  a  pistil  with  an  ovary 


FIG.  68.  Diagrams  showing 
structure  of  cones  in  section. 
A,  male  cone;  B,  female  cone. 
s,  sporophylls  or  spore-leaves  ; 
o,  ovules ;  p,  pollen-sacs ;  e, 
(black)  central  endosperm  part 
of  ovule.  (From  Parker.) 


216  APPLIED  BIOLOGY 

inclosing  the  ovules;  but  in  gymnosperm  cones  the  spore- 
leaves  do  not  unite  to  inclose  the  ovules,  and  hence  these 
ovules  and  the  resulting  seeds  are  said  to  be  "  naked." 

Such  a  cone  containing  spore-cases  or  ovules  on  its  spore- 
leaves  corresponds  in  its  function  to  a  flower  containing  pistils 
only.  Within  each  ovule  is  formed  an  egg-cell  from  which 
an  embryo  develops  after  fertilization.*  This  is  effected  by 
a  cell  which  enters  through  a  tube  (pollen-tube)  from  a 
pollen-grain.  In  short,  fertilization  is  very  similar  to  that 
described  for  plants  with  true  flowers  (§  189). 

Pollination  of  conifers  is  due  to  gravity  and  the  wind,  the 
pollen  falling  on  the  spore-leaves  and  sliding  down  to  the 
ovules.  The  pollen-grains  in  pine  forests  are  often  so  abun- 
dant that  they  cover  the  ground,  the  so-called  "  sulphur  snow." 
The  grains  are  interesting  objects  for  miscroscopic  study,  be- 
cause they  have  two  "  wings,"  which  are  really  air-sacs. 

After  fertilization  is  accomplished  the  fertilized  egg-cell 
in  an  ovule  develops  into  an  embryo,  the  surrounding  cells 
of  the  ovule  develop  into  endosperm  stored  with  food,  and 
the  outermost  cells  form  the  hard  seed-coats.  When  the 
seeds  are  fully  developed  (sometimes  two  years  are  required) 
the  leaves  of  the  cone  dry  and  separate  so  that  the  seeds  may 
fall  out.  The  seeds  of  some  species  of  conifers  have  wings 
which  assist  in  scattering  by  the  wind. 

FRUITS   OF   SEED-PLANTS 

212.  The  word  fruit  in  popular  usage  refers  to  various 
edible  products  of  flowers,  but  in  botanical  terminology  it 
means  the  structure,  usually  containing  seeds,  which  develops 
from  a  flower,  chiefly  from  the  ovary.  In  the  simple  cases 
one  ovule  forms  one  seed,  and  the  wall  of  the  ovary  forms  the 
surrounding  structure.  These  may  be  in  part  edible  (as  in 

*  In  some  gymnosperms  there  are  two  or  more  egg-cells  in  each  ovule 
(as  shown  in  Fig.  61) ;   but  only  one  survives  and  forms  the  embryo  in  the 
which  develops  from  one  ovule. 


STUDIES  OF  SEED-PLANTS  217 

cherry  and  plum),  or  dry  and  hard  (as  in  the  so-called  "  seeds  " 
of  sunflower  and  buckwheat).  In  other  cases,  as  in  bean  pod 
and  poppy  capsule,  there  are  many  seeds  inside  the  structure 
formed  from  the  wall  of  the  ovary  (Fig.  63,  C). 

Some  fruits  are  more  complex  than  the  above  in  that  they 
include  structures  in  addition  to  the  pistil.  For  example, 
the  core  only  of  an  apple  develops  from  the  ovary,  while  the 
outer  edible  fleshy  part  is  formed  by  growth  of  the  receptacle 
which  grows  up  and  surrounds  the  ovary. 

Moreover,  what  is  popularly  termed  a  "  fruit  "  may  con- 
tain several  developed  ovaries  (botanically,  fruits).  For  ex- 
ample, the  fleshy  part  of  a  strawberry  is  formed  from  the 
receptacle  of  the  flower,  and  the  small  hard  grains  (so-called 
"  seeds  ")  on  the  surface  are  each  from  a  pistil  (the  straw- 
berry flower  has  many  pistils),  and  hence  a  strawberry  is  a 
mass  of  aggregated  fruits  set  on  an  edible  receptacle. 

For  purposes  of  study  and  comparison,  it  is  most  conven- 
ient to  group  fruits  under  the  following  headings  :  (1)  simple 
dry  fruits,  (2)  simple  fleshy  fruits,  (3)  stone-fruits,  and  (4) 
complex  fruits.  The  word  "  simple  "  is  used  to  mean  a  fruit 
developed  from  pistil  only,  while  "  complex  "  is  applied  to 
fruits  developed  from  ovary  surrounded  by  overgrown  recep- 
tacle, as  in  case  of  the  apple. 

213.  Simple  Dry  Fruits.  —  Under  this  heading  are  in- 
cluded the  fruits  which  are  developed  from  one  ovary,  and 
in  which  the  ovary-wall  becomes  hardened  and  dry  when 
mature.  There  may  be  in  the  ovary  one  ovule  forming  one 
seed,  or  many  ovules  resulting  in  a  many-seeded  fruit.  Also 
there  may  be  many  pistils  in  a  flower,  and  each  form  a  fruit 
(e.g.,  buttercup).  Examples  of  dry  fruits  are  akenes,  grains, 
nuts,  pods,  follicles,  and  capsules. 

Akene.  —  A  simple  dry  unopening  fruit  (like  that  of  the 
sunflower,  buttercup,  buckwheat,  dock,  bur-marigold)  con- 
taining but  a  single  seed,  is  an  akene.  As  in  the  case  of  the 
sunflower  and  buckwheat,  these  simple  fruits  are  commonly 


218  APPLIED  BIOLOGY 

called  "  seeds/'  but  the  outer  hull  is  from  the  wall  of  ovary 
and  corresponds  to  a  bean  pod,  while  the  seed  with  its  own 
coat  is  inside. 

(L)    Examine  a  buckwheat  or  sunflower  fruit  ("seed"). 

In  the  akene  fruits  of  the  composites  (e.g.,  dandelion),  the 
calyx  usually  adheres  to  the  outer  wall  of  the  ovary,  while 
the  upper  edge  of  the  calyx  appears  on  the  ripened  fruit  as  a 
tuft  of  hairs  which  aid  in  the  dispersal  of  the  seeds. 

Grains. — In  the  unopened  grains  or  fruits  of  the  rice,  oats, 
barley,  etc.,  the  outer  hull  is  formed  from  the  wall  of  the  ovary. 
This  hull  is  entirely  filled  with  the  seed,  and  the  seed-coat  ad- 
heres to  the  ovary-wall. 

Nuts. — The  nuts  of  the  oak  (acorn),  beech,  and  chestnut 
are  true  fruits,  very  much  like  the  akenes,  except  that  usually 
the  dried  ovary- wall  is  very  hard.  The  kernel  inside  is  the 
seed  and  has  its  own  seed-coat. 

In  the  true  edible  chestnuts,  beech-nuts,  and  hazel-nuts, 
two  or  three  nuts  are  within  a  bur  (involucre) ;  but  each  nut 
has  developed  from  one  ovary  (i.e.,  is  a  true  fruit).  In  the 
case  of  the  oak  fruit  (acorn),  the  acorn-cup  surrounds  the  one 
true  fruit  (from  one  ovary) .  The  burs  of  chestnut  and  beech, 
acorn  cups,  and  hazel  hulls  are  formed  from  bracts  of  the  in- 
volucre, which  grow  up  and  surround  the  ovary. 

The  outer  hull  of  walnuts,  hickory  nuts,  and  husks  of  co- 
coanut  develop  from  the  outer  wall  of  the  ovary,  while  the 
inner  wall  forms  the  shell  (see  under  stone-fruits).  The 
Brazil  nut  is  a  seed  with  a  very  hard  seed-coat,  and  a  number 
of  such  seeds  are  inclosed  in  a  hard-walled  shell  which  comes 
from  the  wall  of  the  ovary.  This  is  also  the  case  in  the  horse- 
chestnut  and  buckeye,  where  a  fruit  has  two  seeds  (so-called 
"  nuts  ")  developed  in  a  single  ovary,  and  the  hull  is  formed 
from  the  outer  wall  of  the  ovary. 

Many-seeded  Dry  Fruits.  The  bean  pod  opens  along  both 
edges  or  sutures.  Other  pod-like  fruits  with  many  seeds  open 
along  one  edge  only,  and  are  called  follicles.  The  fruits  of 


STUDIES  OF  SEED-PLANTS  219 

poppy  and  morning-glory  have  several  cavities,  each  with 
seeds,  and  are  called  capsules. 

Aggregates  of  Simple  Dry  Fruits.  We  commonly  call  a  straw- 
berry a  "  fruit,"  but  botanically  considered  it  is  an  aggregate 
of  simple  dry  fruits.  The  true  fruits  of  a  strawberry  are  the 
hard  bodies  (each  with  a  seed  inside)  which  are  set  in  the 
surface  of  the  berry.  Each  of  these  bodies  develops  from  an 
ovary,  and  is  a  single  dry  fruit  or  akene  (§  213).  The  fleshy 
mass  in  which  the  true  fruits  are  embedded  is  the  enlarged 
receptacle.  In  a  strawberry  flower  a  large  number  of  pistils 
are  set  on  a  rounded  receptacle.  Flowers  and  berries  picked 
green  and  preserved  in  formalin  will  help  to  make  the  mode 
of  development  clear. 


FIG.  69.  The  receptacle  of  the  buttercup  flower  bears  many  simple  pistils 
and  each  develops  into  a  simple  dry  fruit  (akene).  &  is  a  section  of  a 
flower  showing  the  globular  receptacle  (white  center). 

The  flower  of  buttercups  produces  a  similar  mass  of  simple 
dry  fruits  (akenes),  but  the  receptacle  does  not  become  as 
much  enlarged  as  in  the  strawberry. 

A  fig  develops  from  a  flower  which  has  numerous  pistils  on 
its  receptacle,  as  in  the  case  of  the  strawberry.  And  in  both 
cases  the  edible  part  is  from  a  receptacle,  but  in  the  fig  flower 
it  is  concave,  whereas  that  of  the  strawberry  is  convex.  As 
a  result  of  this  difference  in  shape  of  the  receptacle,  the  fruits 
of  the  numerous  small  pistils  (the  hard  grains)  are  on  the 
inside  of  the  ripened  fig,  but  on  the  outside  of  the  straw- 


220  APPLIED  BIOLOGY 

berry.  Each  grain  is  simply  a  dry  fruit,  and  contains  the  true 
seed. 

214.  Simple  Fleshy  Fruits.  —  In  all  the  cases  described 
above  the  wall  of  the  ovary  forms  a  hard  or  dry  case  or  shell 
around  the  seed  or  seeds.  In  the  following  fruits  the  ovary- 
wall  becomes  fleshy,  with  a  relatively  thin  external  skin  or 
rind. 

A  date  is  the  simplest  fleshy  fruit,  because  it  contains  but 
one  seed,  which  was  formed  from  a  single  ovule,  while  the 
wall  of  the  ovary  developed  the  edible  fleshy  part  and  the 
thin  rind.  If  one  cracks  a  date  seed,  it  is  found  solid  like  a 
grain  of  wheat,  except  that  the  food  in  wheat  is  starch  while 
in  the  date  seed  it  is  a  hard  woody  substance  (cellulose). 

Tomato.  —  (L)  Materials  :  Small  green  tomatoes  with  short  stems 
attached,  collected  in  the  fall  and  preserved  in  formalin,  will  be  found 
most  satisfactory  for  study  in  the  winter  or  spring.  Some  flowers  in 
formalin  will  also  be  useful.  Note  the  calyx;  how  many  sepals? 
Remove  a  piece  of  skin ;  observe  that  it  is  very  thin  and  tough. 
Make  a  cut  across  the  middle  of  a  tomato  (transverse  section). 
Examine  the  cut  surface;  note  that  the  entire  interior  is  soft  and 
pulpy.  How  many  seed-cavities  do  you  find  ?  They  are  filled  with 
a  soft  placenta  to  which  the  numerous  seeds  are  attached.  Note  the 
gelatinous  covering  of  the  seeds.  Observe  the  thickness  of  the  ovary- 
wall,  and  also  of  the  partitions. 

Other  fruits  like  the  tomato,  in  which  the  entire  wall  of  the  ovary 
becomes  fleshy,  are  the  grape,  cranberry,  egg-plant,  banana,  garden 
pepper,  orange,  lemon,  persimmon,  gooseberry,  currant. 

Lemon.  —  (L)  The  lemon  is  like  a  tomato  in  structure,  but  has  a 
very  tough  leathery  rind.  On  the  stem  end  of  the  fruit  you  may 
find  the  dried-up  remains  of  the  calyx  ;  how  many  sepals  ?  At  the 
opposite  end  is  a  knob-like  projection  to  which  was  attached  the 
stigma  of  the  pistil. 

Remove  the  remains  of  the  calyx  and  observe  the  ring  of  little 
dots  left  in  the  depression  thus  made.  These  are  the  ends  of  the 
fibro-vascular  bundles  which  passed  from  the  stem  (peduncle)  of 
the  flower  up  into  the  pistil.  What  was  their  function? 

The  surface  of  the  lemon  is  roughened  by  numerous  pits  which, 
if  examined  (by  making  thin  sections  of  the  yellow  part  of  the  rind, 
and  placing  under  a  lens)  will  be  found  to  be  little  pits  filled  with  oil, 


STUDIES  OF  SEED-PLANTS  221 

the  oil  of  lemon  from  which  lemon  extract  is  made.  Notice  that  this 
oil  is  very  volatile  and  soon  evaporates.  It  can  be  collected  and 
preserved  by  dissolving  it  in  strong  alcohol,  making  lemon-extract. 
If  a  portion  of  the  white  part  of  the  rind  is  removed  and  very  thin 
cross  sections  are  made,  it  will  be  seen  to  consist  of  layers  of  cells 
very  much  like  the  corky  layer  of  the  potato  skin.  What  is  the 
function  of  this  layer? 

Cut  transversely  across  the  middle  of  a  lemon.  Note  number 
and  shape  of  cavities.  What  relation  exists  between  the  number 
of  seed-cavities  and  the  number  of  knobs  left  in  the  depression  after 
removing  the  calyx  ?  The  cavities  are  entirely  filled  with  a  pulpy 
mass  consisting  of  little  irregular-shaped  tubes  or  bags  containing 
the  juice.  Make  sections  at  various  levels  until  you  find  seeds; 
then  observe  their  arrangement. 

The  gooseberry  and  cranberry  are  simple  fleshy  fruits 
similar  to  lemon  and  tomato. 

Bananas,  navel-oranges,  and  seedless  grapes  are  cultivated 
varieties  in  which  ovules  do  not  develop  into  seeds,  but  the 
ovary-wall  becomes  a  fleshy  fruit  as  in  natural  seeded 
varieties. 

The  pineapple  and  mulberry  are  aggregates  of  fleshy  fruits 
formed  from  numerous  flowers  crowded  on  a  short  flower-stalk, 
which  remains  in  the  center  of  the  ripened  mass.  They  are 
called  multiple  fruits. 

215.  Stone-fruits.  —  Peach,  plum,  cherry,  and  apricot. 
Each  of  these  so-called  "  stone-fruits  "  contains  a  hard  stone- 
like  body  near  the  center  of  the  edible  fleshy  mass,  and  the 
whole  is  inclosed  in  a  thin,  tough  skin.  In  these  fruits,  the 
inner  part  of  the  wall  of  the  ovary  develops  into  a  hard,  stony 
substance  inclosing  the  seed  and  the  outer  part  of  the  ovary- 
wall  forms  a  pulpy  edible  tissue.  Inside  each  "  stone  "  or 
"  pit  "  is  a  true  seed  (often  called  the  kernel)  with  its  own 
seed-coat.  This  kernel  corresponds  to  a  date  seed,  but 
the  ovary-wall  of  the  date  forms  fleshy  substance  close  around 
the  seed  and  no  hard  stony  shell  as  in  the  peach  and  the  plum. 

The  walnuts  and  hickory-nuts  are  similar  to  stone-fruits, 
except  that  their  outer  fleshy  part  gets  hard  late  in  the  sum- 


222  APPLIED  BIOLOGY 

mer  and  becomes  the  "  hull  "  or  "  shuck."  Before  this 
hardening  occurs,  young  English  walnuts  are  sometimes  used 
for  making  pickles.  Obviously,  a  hulled  walnut  or  hickory- 
nut  corresponds  in  origin  to  a  peach  or  plum  stone. 

(L)  Examine  any  stone-fruits  available.  Also  flowers  and 
young  stages  preserved  in  formalin. 

Aggregates  of  Stone-fruits.  —  Raspberry  and  blackberry. 
Each  of  the  small  rounded  bodies  of  which  these  berries 
are  composed  is  a  stone-fruit,  developed  from  an  ovary.  The 
whole  berry  is  an  aggregate  of  fleshy  fruits ;  and  each  one  of 
these  bodies  corresponds  to  a  cherry.  The  numerous  pistils  of 
the  flowers  are  crowded  so  closely  together  that  the  resulting 
fruits  become  united  into  the  one  mass  which  we  know  as 
a  blackberry  or  raspberry.  This  mass  when  ripe  separates 
easily  from  the  receptacle.  In  short,  a  blackberry  or  rasp- 
berry is  equivalent  to  many  small  cherries  which  we  might 
imagine  growing  together  into  one  solid  mass  set  on  one  large 
receptacle.  Blackberry  flowers  and  green  berries  preserved 
in  formalin  should  be  examined. 

216.  Complex  Fleshy  Fruits  are  formed  from  an  ovary 
plus  some  adjoining  flower  parts.  All  the  fruits  described 
in  the  preceding  sections  are  formed  from  the  ovary  only. 
In  some  complex  cases  the  receptacle  grows  up  around  the 
ovary  as  in  Fig.  63,  C;  the  ovary  therefore  appears  to  be 
below  the  calyx  and  corolla  (inferior  ovary),  and  the  fruit 
formed  is  composed  of  ovary  and  thickened  receptacle  (apple 
and  cucumber). 

Apple.  —  (L)  Materials :  Apples,  apple  flowers,  and  various  stages 
of  the  young  fruit  in  formalin.  Optional,  pears  and  quinces. 

Note  the  dried-up  ends  of  sepals  at  end  opposite  the  stem.  Com- 
pare series  of  stages  from  flower  to  the  young  fruit.  Observe  the 
waxy  coating  on  the  skin.  Suggest  use.  Remove  the  skin  from  an 
apple,  put  aside  the  peeled  apple  and  examine  it  after  a  day  or  two. 
Use  of  the  skin? 

Cut  the  apple  crosswise  midway  between  the  stem  and  sepal 
ends.  Note  the  star-like  arrangement  of  the  seed-cavities.  How 


STUDIES  OF  SEED-PLANTS  223 

many?  Where  are  the  seeds  attached,  and  how  many  in  each 
cavity?  Note  the  tough  papery  walls  of  these  cavities;  this  is 
the  ovary-wall,  commonly  called  the  core  of  the  apple.  The  fleshy 
mass  which  forms  the  bulk  of  the  edible  part  is  really  the  thickened 
receptacle  of  the  flower.  In  the  fleshy  part,  a  short  distance  from 
the  seed-cavities,  is  a  ring  of  dots,  the  cut-off  ends  of  the  fibro- 
vascular  bundles  which  carried  the  food  into  the  developing  apple. 
How  many  dots  are  there  and  what  relation  is  there  between  their 
number  and  that  of  the  seed-cavities  and  sepals  ?  In  a  longitudinal 
section  of  apple  cut  from  stem  end  to  sepal  end,  the  string-like  nature 
of  these  nbro-vascular  bundles  will  be  more  evident ;  they  can  be 
pulled  out  much  as  you  removed  the  strings  from  the  pod  of  the  bean 
in  an  earlier  lesson.  Note  that  these  bundles  originate  in  the  stem 
of  the  apple  and  pass  up  to  the  sepals. 

Compare  (optional)  the  fruits  of  pear,  quince,  and  apple. 

Cucumber  or  Squash,  Melon,  Gourd.  —  Examine  a  series  of  stages 
showing  development  of  cucumber  flower  into  the  fruit.  These 
may  be  collected  in  summer  and  preserved  in  formalin.  Cut  a  cu- 
cumber crosswise.  How  many  seed-cavities  do  you  find?  Where 
are  the  seeds  attached?  The  rind  and  the  fleshy  mass  outside  of 
the  seed-chambers  is  derived  from  ovary-wall  and  receptacle. 
Examine  a  cross  section  of  an  immature  squash ;  observe  the  seed- 
cavities  in  the  center,  and  the  attachment  of  the  seeds  to  placentas. 
The  region  immediately  outside  the  seed-cavities  is  the  ovary-wall, 
the  remainder  of  the  fleshy  mass  and  the  rind  is  derived  from  the 
receptacle.  In  the  watermelon  the  main  bulk  and  the  edible  part 
is  the  enlarged  placenta. 

*  217.  Dispersal  of  Fruits  and  Seeds.  —  It  is  an  interesting 
fact  that  only  by  the  scattering  of  fruits  and  seeds  is  it  pos- 
sible for  most  higher  plants  to  become  widely  distributed.  A 
single  plant  of  the  Russian  thistle  may  produce  several  hun- 
dred thousand  seeds.  If  all  these  seeds  were  to  fall  at  the 
base  of  the  plant  and  there  start  to  develop,  it  is  very  likely 
that  none  of  them  would  reach  maturity,  for  they  would 
crowd  and  starve  each  other  to  death.  Nature  has  pro- 
vided against  such  a  calamity  and  ensured  the  distribution 
of  the  species,  for  the  plant  when  mature  assumes  a  ball-like 
form,  breaks  off  at  the  ground,  and  is  rolled  by  the  wind  for 
miles  from  the  place  where  it  grew,  dropping  its  seeds  as 


224  APPLIED  BIOLOGY 

it  is  tumbled  along.  This  is  only  one  of  many  methods  of 
seed  dispersal.  The  success  of  the  common  dandelion  as 
a  weed  depends  in  a  large  measure  upon  its  efficient  device 
for  scattering  its  seeds.  Here,  as  in  many  other  composites, 
the  upper  end  of  the  calyx  forms  a  tuft  of  hairs  which  are 
caught  by  the  wind.  In  still  other  plants  like  the  clematis 
the  style  of  the  pistil  becomes  a  feathery  tail  by  which  the  seed 
is  wafted  about.  In  the  milkweed  the  little  seed-stalk  de- 
velopes  into  a  tuft  of  silky  hairs  so  light  that  the  slightest 
breeze  may  carry  the  seeds  miles  away  from  the  parent 
plant.  Plants  like  the  elm,  maple,  ailanthus,  and  pine,  also 
depend  upon  the  wind  to  scatter  their  seeds;  but  the  simple 
wing-like  outgrowths  from  these  seeds  and  fruits  do  not  make 
them  very  buoyant,  and  consequently  they  are  not  carried 
far  by  the  wind.  In  edible  fruits  like  the  wild  blueberry, 
pokeberry,  currant,  etc.,  the  seeds  are  so  hard  that  probably 
they  are  not  injured  by  being  passed  through  the  digestive 
organs  of  the  birds  or  other  animals  that  feed  upon  the  fruits, 
and  hence  fruit-eating  animals  may  serve  as  distributers  of 
hard  seeds  far  from  the  locality  where  they  grew.  In  such 
cases  as  the  black  walnut,  wild  apple,  osage-o range,  etc.,  the 
fruits,  because  of  their  rotund  form,  may  roll  some  distance 
from  the  parent  tree.  In  some  plants  (e.g.,  witch  hazel) 
the  seeds  are  inclosed  in  an  explosive  pod  which  when  it 
opens  throws  the  seeds  many  feet  away.  The  jewel- weed, 
garden  balsam,  violet,  and  morning-glory  also  belong  to  this 
type.  In  some  plants  the  seeds  or  fruits  develop  hooks  and 
barbs  which  catch  hold  of  the  furs  of  passing  animals  or  the 
clothing  of  people  and  are  thus  widely  scattered.  Still  others 
have  their  seeds  or  fruits  inclosed  in  waterproof  husks  or 
envelopes  so  that  they  may  be  carried  for  miles  by  water,  and 
as  in  the  case  of  the  cocoanut  fruit,  probably  thousands  of 
miles.  It  is  probably  in  this  way  that  the  flora  (collection 
of  plants)  of  isolated  coral  islands  originated.  Seeds  are  also 
scattered  in  accidental  ways,  as  for  instance  in  the  pellets 


STUDIES  OF  SEED-PLANTS  225 

of  mud  on  the  feet  of  wading  birds,  and  nuts  buried  or  hidden 
away  by  birds  or  squirrels  and  then  never  eaten. 

Students  of  botany  should  guard  against  the  common 
tendency  to  look  upon  adaptations  for  dispersal  of  seeds  as 
having  developed  because  plants  required  them.  That  this 
is  not  true  is  indicated  by  the  fact  that  numerous  species  of 
plants  have  no  obvious  special  methods  for  distributing  seeds. 
All  that  we  are  justified  in  stating  is  that  plants  with  seeds 
specially  adapted  for  dissemination  have  a  better  chance  of 
spreading  over  a  large  territory.  We  do  not  know  why  or 
how  plants  acquired  their  peculiar  methods  of  seed-dispersal. 

SEED-PLANTS  REPRODUCING  WITHOUT  FLOWERS 
AND  SEEDS 

218.  While  the  flowering  plants  naturally  reproduce  from 
seeds,  many  of  them  may  also  reproduce  by  other  methods. 
Many  cultivated  plants  are  examples.  Strawberry  plants  are 
commonly  obtained  from  branches  (called  runners)  which 
creep  over  the  ground  and  take  root  at  the  joints  or  nodes, 
and  form  new  plants  at  these  points.  In  this  way  one  plant 
has  formed  more  than  fifty  new  plants  in  a  single  summer. 
Black  raspberry  bushes  and  grape-vines  bend  over,  form 
roots,  and  develop  new  plants  where  they  touch  the  ground. 
Red  raspberry  bushes  multiply  by  forming  young  plants 
from  underground  branches  resembling  roots.  New  plants 
start  from  roots  of  osage-orange  and  some  other  trees.  Some 
flowering  plants  may  reproduce  from  leaves;  the  best  ex- 
ample is  the  "  sprout-leaf  "  or  bryophyllum,  a  garden  plant 
whose  leaves  fall  to  the  ground,  form  roots  and  shoots  at  the 
notches  on  edges  of  the  leaves,  and  thus  each  leaf  may  form 
a  number  of  new  plants.  Some  begonia  leaves  will  propagate 
in  the  same  way.  Still  other  flowering  plants  propagate 
from  bulbs  (which  are  short  stems  with  leaves).  This  is  well 
illustrated  by  certain  kinds  or  varieties  of  onions.  The  com- 
mon onions  ("  seed  onions  ")  are  grown  from  seed  in  one 
Q 


226  APPLIED  BIOLOGY 

season,  or  else  the  seeds  are  started  one  year  and  form  small 
onions  ("  sets  ")  which  are  planted  the  next  spring.  In  this 
case  one  seed  forms  one  set,  and  one  set  forms  one  onion  bulb, 
which  when  mature  forms  flowers  and  seeds ;  hence  multiplica- 
tion can  be  only  by  seeds.  Some  other  kinds  of  onion  bulbs 
behave  differently  in  that  the  bulb  which  is  planted  divides 
into  a  group  of  little  bulbs  (bulbels).  The  multiplier  or 
potato  onion  does  this,  and  garlic  is  similar.  Each  of  the 
little  bulbels  may  be  planted  and  will  soon  grow  to  full  size, 
and  then  begin  to  form  more  bulbels.  Multiplier  onions 
occasionally  produce  seed,  but  gardeners  commonly  use  the 
bulbels  for  planting.  A  third  kind  of  onions  form  "  top 
onions  "  or  bulblets  on  the  flower-stalks,  sometimes  mixed 
with  some  flowers.  These  small  bulblets  may  be  planted 
next  season  and  soon  grow  to  usable  size,  but  if  left  in  the 
ground,  they  will  send  up  flower-stalks  which  produce  bulblets. 
The  so-called  "  Egyptian  onions  "  and  "  tree  onions  "  are 
top-onions.  In  the  wild  garlic  some  of  the  flowers  are  fre- 
quently replaced  by  bulblets.  The  above  ways  of  growing 
different  varieties  of  onions  illustrate  the  methods  of  propa- 
gating many  bulb-producing  plants.  Hyacinth,  narcissus 
or  daffodil,  many  lilies,  crocus,  and  tulip  are  examples  of 
flowering  plants  which  are  usually  grown  from  bulbs  and 
rarely  from  seed. 

Without  the  aid  of  man  some  flowering  plants  may  propa- 
gate from  roots,  stems,  leaves,  bulbs,  in  addition  to  seeds. 
By  man's  help  many  of  them  which  do  not  naturally  propa- 
gate without  seeds  can  grow  from  parts  of  the  plants  like 
stems  and  roots.  For  example,  cuttings  or  slips  may  be  cut 
from  stems  of  almost  any  plant  and  if  properly  set  in  moist 
soil  will  form  roots  and  develop  into  a  new  complete  plant. 
This  is  the  familiar  method  of  starting  geraniums,  coleus,  and 
other  house-plants;  and  dozens  of  shrubs  (e.g.,  currant), 
vines  (e.g.,  grape),  and  trees  (e.g.,  willow)  are  commonly 
started  from  cuttings  of  stems.  Go  into  any  greenhouse 


STUDIES  OF  SEED-PLANTS  227 

and  one  will  find  a  sand-bed  used  for  starting  roots  on  such 
cuttings. 

Still  another  artificial  method  of  propagation  from  cuttings 
is  in  the  processes  called  grafting  and  budding.  The  details 
of  this  method  are  explained  in  §  165,  but  essentially  it  con- 
sists in  taking  a  small  piece  of  stem,  with  buds  or  a  single  bud, 
from  one  plant  and  attaching  it  to  root  or  stem  of  another 
plant.  The  transplanted  buds  grow,  and  when  placed  near 
the  root  will  form  the  entire  plant  above  ground.  This  is  the 
method  for  obtaining  standard  varieties  of  apples,  pears, 
plums,  cherries,  peaches,  and  many  other  shrubs  and  trees. 

All  such  propagation  of  plants  from  parts  or  pieces  of  full- 
grown  plants  without  seeds  is  known  as  asexual  or  vegetative 
reproduction.  The  propagation  of  plants  by  means  of  seeds  in- 
volves sexual  reproduction,  for  most  seeds  originate  through 
fertilization.  As  we  have  seen,  in  some  cultivated  plants  the 
method  of  reproducing  by  seeds  has  been  supplanted  largely 
by  asexual  reproduction.  This  has  two  advantages  :  (1)  the 
ease  of  starting  some  plants  without  seeds,  and  (2)  many 
cultivated  varieties  will  always  produce  their  own  variety 
from  parts  of  themselves,  but  not  from  seeds.  For  example, 
the  seeds  from  a  red  apple  may  grow  into  trees  which  will 
bear  green-colored  apples  and  entirely  different  in  flavor,  size, 
etc.  The  only  way  to  propagate  with  certainty  the  desired 
variety  is  to  take  a  cutting  from  a  branch  of  the  apple  tree 
known  to  bear  the  kind  of  fruit  wanted  and  graft  it  on  a  tree 
started  from  seed.  The  runners  of  a  strawberry  plant  will 
form  new  plants  producing  the  same  quality  of  berries,  but 
the  seeds  rarely  do  so.  Moreover,  in  the  case  of  seedless 
fruits  (e.g.,  navel  oranges  and  seedless  apples)  the  only  possible 
propagation  is  by  cuttings,  buds,  or  grafts.  The  millions  of 
seedless  orange  trees  in  California  have  descended  from  a 
single  tree  which  grew  from  seed  but  could  not  itself  produce 
seed.  Without  man's  help  such  a  variety  could  not  have 
multiplied. 


228  APPLIED  BIOLOGY 


GENERAL  NOTES   ON  SEED-PLANTS 

219.  Adaptations  of  Seed-plants.  —  The  preceding  lessons 
on  seeds,  roots,  stems,  flowers,  and  fruits  of  seed-plantshave 
shown  some  of  the  most  important  ways  in  which  these  organs 
have  become  adaptively  modified  so  as  to  enable  certain 
plants  to  carry  on  their  life-activities  (especially  breathing, 
feeding,  and  reproducing)  better  than  an  unmodified  plant 
could  under  the  same  conditions.  Write  a  brief  essay  on 
"  Adaptations  of  Seed-plants,"  showing  how  at  least  five 
adaptations  have  given  plants  which  possess  them  special 
advantage. 

Origin  of  the  Adaptations.  —  Just  how  adaptations  of  the 
various  organs  of  seed-plants  have  originated  is  unknown  to 
biologists ;  but  the  fact  that  modified  roots,  stems,  flowers,  etc., 
are  built  on  the  plan  of  unmodified  ones  suggests  that  origi- 
nally they  were  all  alike.  For  example,  it  seems  probable 
that  the  first  flowers  were  very  simple,  and  that  their  de- 
scendants have  developed  various  modifications  which  adapt 
them  to  such  important  processes  as  pollination  and  seed- 
distribution.  However,  it  does  not  seem  probable  that 
adaptations  have  been  developed  because  they  were  needed ; 
but  rather  that  modified  structures  have  appeared  for  some 
cause  or  reason  unknown  to  us,  and  have  happened  to  be  of 
special  use  or  value.  For  example,  it  seems  probable  that 
certain  leaves  first  developed  in  pitcher-form,  not  because  the 
plants  required  such  leaves  and  set  about  to  make  them;  but 
having  appeared  as  the  result  of  causes  still  unknown,  the 
pitchers  proved  useful  as  insect-catchers  and  have  been  pre- 
served, and  probably  improved. 

This  idea  that  many  structures  developed  and  then  later 
found  a  special  use  and  became  adaptations  is  supported  by 
the  observation  that  many  other  modified  structures  have 
apparently  found  no  use.  For  example,  it  is  not  known  that 
the  sharp  spines  on  a  chestnut-bur  are  useful  defenses,  for  when 


STUDIES   OF  SEED-PLANTS  229 

the  nuts  are  ripe  and  attractive  to  squirrels  the  burs  split 
open  and  scatter  the  nuts  on  the  ground  beneath  the  trees. 
Obviously  the  spines  were  not  made  to  protect  the  nuts  from 
squirrels,  as  some  books  assume,  for  the  spines  are  not  hard 
until  the  nuts  are  ripening,  and  before  that  time  no  squirrel 
would  care  for  them.  What  then  is  the  explanation?  It 
appears  to  be  this,  that  like  other  modified  structures,  the 
spines  originally  appeared  without  any  regard  to  possible 
use,  and  the  splitting  habit  of  the  chestnut-bur  has  made  it 
impossible  for  them  to  become  useful.  If  such  spines  had 
developed  on  fruits  which  do  not  split  and  discharge  their 
seeds,  we  can  imagine  that  they  might  have  been  of  some 
advantage  and  have  become  a  useful  adaptation  as  a  pro- 
tection against  gnawing  animals.  There  are  many  similar 
cases  among  animals  and  plants  where  structures  may  at  first 
sight  appear  to  have  a  use,  but  closer  study  shows  us  that 
they  are  not  useful  adaptations. 

If  structures  appear  without  any  reference  to  use,  and  prove 
to  be  actually  harmful,  the  plants  which  possess  them  will 
probably  disappear  in  time.  For  example,  if  the  spines  on 
some  chestnut-burs  had  in  any  way  interfered  with  the 
complete  development  of  seeds,  the  result  would  have  been 
that  the  trees  having  such  harmful  spines  would  long  ago 
have  disappeared,  while  the  trees  without  them  would  have 
perpetuated  their  kind,  and  we  should  now  have  only  trees 
producing  spineless  chestnut-burs.  But  the  fact  that  the 
spines  continue  to  exist  without  serving  any  use  suggests 
that  they  are  neither  harmful  nor  useful,  but  simply  harm- 
less or  neutral.  There  are  many  such  harmless  structures 
among  animals  and  plants. 

The  above  discussion  will  suggest  that  the  student  of 
biology  who  is  interested  in  adaptations  of  animals  and  plants 
must  not  expect  to  find  an  evident  use  for  everything.  Appar- 
ently some  things  have  use,  and  many  things  have  none 
evident  at  the  present  time.  In  some  of  the  apparently  use- 


230  APPLIED  BIOLOGY 

less  cases  there  are  reasons  for  believing  that  under  different 
conditions  of  past  times  some  things  not  now  useful  may 
have  had  a  use. 

The  tendency  to  look  for  usefulness  in  everything  is 
strong  in  the  human  mind.  Long  ago  all  nature  was  looked 
upon  as  made  for  the  use  of  man ;  plants  for  man  to  eat  and 
use  in  other  ways,  and  animals  for  human  food,  clothing, 
beasts  of  burden,  pets,  etc.  And,  after  all,  it  is  not  very  sur- 
prising that  men  got  into  this  way  of  thinking,  for  so  many 
animals  and  plants  are  useful  to  us.  This  fact  is  impressed 
upon  the  reader  of  such  books  as  Shaler's  "  Domesticated 
Animals,'7  Wood's  "  Dominion  of  Man,"  and  books  dealing 
with  useful  plants.  But  within  the  past  fifty  years  this 
attitude  has  changed,  and  now  scientific  men  do  not  look 
upon  all  nature  as  having  been  made  for  the  direct  use  of 
man,  but  believe  that  animals  and  plants  have  a  reason  for 
existence  even  if  there  were  no  human  beings  in  the  world. 
In  fact,  geology  has  shown  that  vast  numbers  of  organisms 
lived  on  this  earth  millions  of  years  before  man  appeared. 

The  passing  of  the  view  that  all  the  world  is  arranged  with 
direct  reference  to  use  by  man  has  been  followed  by  the  tend- 
ency to  look  upon  all  things  in  each  animal  and  plant  as 
useful  to  itself,  to  its  offspring,  or  to  its  kind  or  species.  This 
also  was  the  outcome  of  the  truth  that  a  large  number  of 
things  are  obviously  useful.  There  can  be  no  doubt  concern- 
ing the  usefulness  of  some  adaptations  of  flowers  to  insects, 
of  plants  to  storage  of  food,  of  leaves  in  the  light-relation, 
and  of  some  seeds  for  distribution ;  but  no  one  knows  a  use 
for  many  other  things  (e.g.,  prickles,  thorns,  hairs,  and  the 
brilliant  colors  in  many  plants,  or  the  fleshy  parts  of  some 
inedible  fruits).  Hence  science  does  not  warrant  the  con- 
clusion that,  even  if  many  things  are  not  arranged  for 
the  use  of  man,  they  are  all  of  use  to  the  organisms  which 
possess  them.  The  important  point  in  the  whole  matter  is 
the  fact  that  many,  very  many,  modified  structures  are 


STUDIES  OF  SEED-PLANTS  231 

surely  adaptations;  that  is,  fitted  to  special  use ;  and  so,  while 
the  student  of  biology  is  kept  constantly  on  the  lookout  for 
evidence  of  such  useful  structures,  he  must  keep  in  mind 
that  now  and  then  there  will  be  found  things  without  appar- 
ent use.  Nature  has  no  hard  and  fast  rule  that  all  things 
must  be  useful  as  seen  from  our  human  point  of  view.  A  sur- 
prisingly large  number  of  things  are  useful  to  the  organisms 
which  possess  them,  but  there  are  many  puzzling  exceptions. 
220.  The  economic  importance  of  the  seed-plants  is  so 
vast  that  a  special  book  would  be  required  to  discuss  it 
adequately.  Here  we  can  only  point  to  the  fact  that  the  great 
majority  of  plants  useful  to  man  are  seed-plants.  Practi- 
cally all  of  our  plant  food-supply  and  that  of  our  domesticated 
birds  and  mammals ;  all  of  our  forests  useful  for  lumber ;  all 
of  our  plants  which  produce  fibers  (cotton,  hemp,  linen,  etc.) ; 
most  of  the  plants  which  produce  drugs  and  other  special 
substances ;  and  almost  all  of  our  ornamental  plants  belong 
in  the  great  group  of  seed-plants  or  flowering  plants.  The 
mere  mention  of  food  plants,  fiber  plants,  and  lumber,  calls 
to  mind  the  vast  agricultural  and  manufacturing  industries 
which  have  been  built  up  on  the  basis  of  seed-plants.  In 
fact,  the  foundation  of  the  wealth  of  the  civilized  nations  is 
in  agriculture,  which  is  primarily  the  business  of  producing 
useful  seed-plants.  With  so  much  to  the  credit  of  the  seed- 
plants,  there  seems  to  be  little  remaining  chance  for  useful- 
ness of  lower  plants ;  and  yet  we  shall  see  that  many  of  them 
have  special  useful  relations  to  human  life. 


CHAPTER  IX 
STUDIES  OF  SPORE-PLANTS 

221.  Seeds  and  Spores.  —  All  the  plants  mentioned  in 
the  preceding  chapters  are  characterized  by  the  development 
of  seeds  containing  embryo  plants,  and  hence  such  plants 
are  named  "  seed-plants."  But  all  plants  do  not  produce 
seeds  with  embryos  ready  to  unfold  when  placed  in  proper 
conditions  for  germination.  There  are  a  number  of  types 
of  lower  plants  without  seeds,  but  which  form  spores,  simple 
rounded  bodies  without  embryos  but  able  to  develop  into 
new  plants.  Such  plants  are  called  "  spore-plants. "  Ferns, 
mosses,  and  mushrooms  are  examples.  It  is  true  that  in  a 
seedsman's  catalogue  we  may  find  "  fern  seeds  "  listed ;  but 
this  is  a  careless  use  of  the  word  "  seed/'  for  what  the  seeds- 
man sells  are  really  fern  spores. 

However,  while  spore-plants  do  not  form  seeds,  seed-plants 
do  have  spores.  These  are  of  two  kinds :  (1)  the  pollen- 
grains  (microspores,  little  spores),  and  (2)  a  mass  (megaspore, 
great  spore)  inside  each  ovule  from  which  the  embryo 
(which  is  at  first  egg-cell)  and  endosperm  of  the  seed  develop. 
We  see  that  both  seed-plants  and  spore-plants  produce 
spores.  The  chief  difference  is  that  in  all  seed-plants  the 
spores  develop  inside  the  ovary  into  embryos  (and  endosperm, 
if  present  in  seed) ;  while  in  spore-plants  the  spores  may  be 
scattered  far  from  the  plant  which  produced  them  and  then 
germinate  into  new  plants.  We  shall  be  able  to  understand 
this  point  better  after  studying  briefly  the  life-history  of  a 
fern  plant. 

232 


STUDIES  OF  SPORE-PLANTS  233 

222.  Cryptogams   and   Phanerogams.  —  Spore-plants  are 
often  called   cryptogams,  which  means  hidden  reproduction, 
and  was  given  when  their  life-histories  were  not  well  under- 
stood.    The    term    phanerogams,    applied    to    seed-plants, 
means  evident  reproduction,  and  was  applied  because  the 
conspicuous  flowers  are  obviously  connected  with  reproduc- 
tion. 

I.   HIGHER  SPORE-PLANTS:    FERNS  AND  MOSSES 

The  ferns  and  mosses,  and  their  relatives,  are  distinguished 
by  the  possession  of  structures  which  are  similar  to  and  per- 
form the  functions  of  the  roots,  stems,  and  leaves  of  seed- 
plants.  These  are  lacking  in  the  lower  spore-plants  (§  235). 

FERNS 

223.  General  Structure  of  a  Fern  Plant.*  —  Any  common 
kind  or  species  of  ferns  may  serve  as  material  for  illustrating 
this  study  and  for  parallel  laboratory  work.     The  fern  plants 
are  like  the  seed-plants  in  having  roots,  stem,  and   leaves 
(also  called  fronds).     The  stem  of  our  ordinary  ferns  is  com- 
monly on  the  surface  or  slightly  buried  in  the  soil ;  but  the 
tree-ferns  of  tropical  countries,  often  seen  in  greenhouses, 
have   upright   stems.     The   prostrate   stem    (rootstock)    of 
ordinary  ferns  grows  from  a  terminal  bud,  and  since  the  stem 
lives  for  many  years  and  the  leaves  usually  only  one  summer, 
the  stem  is  continually  advancing  and  sending  up  new  leaves 
farther  and  farther  away  from  the  position  of  the  first  ones. 
The  stem  may  branch ;  and  since  the  older  part  of  the  stem 
dies  and  decays,  it  frequently  happens  that  the  branches  get 
separated  from  the  main  stem  and  become  separate  plants. 

The  root,  stem,  and  leaf  of  ferns  are  similar  in  structure  to 
those  of  seed-plants.  In  the  stems  are  vascular  bundles  con- 


*  Concerning  laboratory  work  on  ferns :  The  teacher  should  provide 
the  students  with  materials  for  demonstrations  or  laboratory  study  of  the  main 
points  in  the  following  description  of  ferns.  See  "Teacher's  Manual." 


234 


APPLIED  BIOLOGY 


taining  wood-tubes  and  sieve-tubes,  which  act  as  in  seed- 
plants  in  conducting  fluids  up  and  down  the  stem.  Also,  as 
in  the  higher  plants,  the  vascular  bundles  extend  into  the 
veining  of  the  leaf,  thus  providing  the  leaf  with  a  water- 


FIG.  70.  Aspidium  fern.  A,  part  of  underground  stem  with  three  young 
leaves  (a),  and  parts  of  three  mature  leaves,  one  with  numerous  clusters 
of  spore-cases.  B,  section  transverse  of  leaflet  with  cluster  of  spore-cases 
(c)  on  lower  side  and  covered  by  a  cap  (6).  C,  under  side  of  a  leaflet 
with  seven  clusters  of  spore-cases.  (From  Strasburger.) 

supply  system  and  a  means  of  transferring  elaborated  foods 
to  cells  of  the  stem  and  roots.  The  general  form  and  struc- 
ture of  the  leaves  reminds  one  of  those  of  seed-plants.  The 
leaves  of  many  ferns  are  much  divided  and  resemble  com- 
pound leaves  of  seed-plants. 

The  one  most  striking  difference  as  compared  with  the 


STUDIES   OF  SPORE-PLANTS 


235 


leaves  of  seed-plants  is  that  fern  leaves  may  have  on  their 
lower  surface  peculiar  organs  which  form  spores.  In  some 
species  of  ferns  there  are  on  the  leaflets  rounded  spots ;  in 
others  there  are  ridges  on  the  leaflets  ;  and  in  still  others  there 
are  folds  at  the  margin.  Examination  with  a  hand-lens  shows 
that  these  variously  shaped  spots  are  similar  in  essential 
structure,  for  they  are  clusters  of  spore- 
cases  (sporangia),  the  organs  for  pro- 
ducing spores.  When  immature,  these 
spore-cases  are  usually  protected  by  a 
covering  (called  indusium)  in  the  form  of 
a  cap,  or  a  fold  of  the  edge  of  the  leaf. 

In  many  ferns  the  leaves  thus  combine 
the  regular  work  of  foliage  leaves  (breath- 
ing, transpiration,  starch-making)  and 
that  of  spore  forming ;  but  some  species 
have  certain  specialized  leaves  which  pro- 
duce spores  (spore-leaves  or  sporophylls) , 
and  thus  the  foliage  leaves  are  free  for  the 
regular  work.  In  such  cases  the  foliage 
leaves  are  often  called  sterile  leaves,  the 
spore-bearing  ones  fertile.  In  some  spe- 
cies these  latter  become  so  much  modified  FIG.  71. 
that  they  do  not  resemble  ordinary  leaves 
(Fig.  71). 

224.    Germination  of   Fern  Spores.  - 
When  the  spore-cases  (sporangia)  are  ma- 
ture and  dry,  they  burst  and  scatter  the  spores. 


Specialized 
spore-bearing  fern 
leaf  (6),  leaflet  mag- 
nified (c) ,  and  regular 
leaf  (a).  (From 
Gray.) 


The  burst- 
ing is  caused  by  a  peculiar  ring  (annulus)  which  almost 
surrounds  each  spore-case  and  acts  like  a  spring.  If  the 
spores  fall  upon  favorable  soil,  they  absorb  water  and  ger- 
minate. The  coats  of  the  spore  burst  and  a  delicate  tube 
protrudes  (Fig.  72,  A).  This  soon  divides  into  cells  (Fig. 
72,  B),  and  grows  into  a  flat  and  usually  heart-shaped 
structure,  called  a  prothallium  (C).  On  its  under  side  are 


236  APPLIED  BIOLOGY 

root-hairs  (rhizoids),  which  act  like  true  roots  of  seed-plants 
in  absorbing  from  the  soil.  The  cells  of  the  prothallium 
are  green  with  chlorophyll-bodies,  which  are  able  to  make 
starch  as  do  ordinary  leaves.  In  fact,  the  great  growth 
from  a  minute  spore  to  a  prothallium,  which  may  be  more 
than  one  eighth  inch  in  diameter,  is  due  largely  to  the 
materials  made  by  the  chlorophyll-bodies,  supplemented  by 
some  mineral  and  nitrogenous  materials  absorbed  by  the 
root-hairs  from  the  soil. 

In  order  to  get  prothallia  for  study,  one  should  visit  green- 
houses where  ferns  are  grown  from  spores.  Sometimes  they 
will  be  found  on  the  surface  of  the  soil  beneath  ferns  kept  in 
moist  atmosphere,  as  in  a  fernery  (a  glass-covered  case  for 
keeping  plants  in  a  moist  atmosphere). 

225.  Sex-organs  of  the  Prothallium.  —  In  most  species  of 
ferns,  each  prothallium  develops  on  its  under  side  both  male 
and  female  sex-organs  for  the  production  of  egg-cells  and 
sperm-cells.  Each  female  organ  (ovary,  or  usually  called 
archegonium  in  ferns  and  mosses)  is  a  tube-like  structure 
(Fig.  72,  F),  containing  in  the  expanded  base  of  the  tube 
an  egg-cell  or  ovum.  Each  male  organ  (spermary,  or  an- 
theridium)  is  a  spherical  body,  in  whose  central  cavity  are 
produced  many  spiral-shaped  cells  with  a  group  of  cilia 
(Fig.  72,  D,  E) .  These  are  the  sperm-cells  (also  called  sper- 
matozoids),  and  the  lashing  movements  of  their  cilia  cause 
them  to  swim  in  drops  of  water  on  the  under  side  of  the  pro- 
thallium  to  the  mouth  of  an  ovary.  The  tube  or  neck  of 
the  ovary  is  filled  with  a  mucilage-like  material,  through 
which  the  sperm-cells  swim  to  the  egg-cell.  One  sperm- 
cell  unites  with  an  egg-cell,  thus  accomplishing  fertiliza- 
tion. Notice  the  similarity  to  the  fertilization  of  the  frog's 
eggs  (§  58). 

Compared  with  fertilization  in  seed-plants  (described  in 
§  189),  the  sperm-cell  of  the  fern  obviously  has  the  same 
function  as  the  cell  which  moves  down  the  pollen-tube  to 


STUDIES   OF  SPORE-PLANTS 


237 


FIG.  72.  A,  fern  spore  germinating;  s,  spore;  rh,  rhizoid.  B,  a  later  stage  of 
germination;  ap.c,  apical  cell  or  growing  point.  C,  full-grown  pro- 
thallium,  seen  from  the  lower  side,  many  rhizoids,  ovaries  (ov),  sper- 
maries  (sp).  D,  section  of  spermary,  immature  sperm-cells  in  center  ; 
unshaded  cell  belongs  to  the  prothallium,  beneath  which  spermaries  are 
suspended.  E,  a  sperm-cell,  coiled  body,  and  a  group  of  cilia  for  use  in 
swimming.  F,  section  of  an  ovary,  with  egg-cell  (o).  The  wide-open 
neck  of  the  ovary  is  in  nature  filled  with  a  mucilage  through  which  sperm- 
cells  swim  to  the  ovum.  G,  prothallium  with  a  young  fern  plant  de- 
veloped from  a  fertilized  ovum  in  an  ovary,  rh,  rhizoids  ot  the  pro- 
thallium;  rt,  root  of  the  young  plant;  ct,  a  cotyledon;  st,  beginning  stem; 
I,  first  true  leaf  forming.  (From  Parker's  Biology,  with  modifications.) 


238  APPLIED  BIOLOGY 

accomplish  fertilization  of  the  egg-cell  in  an  ovule  of  a  flower. 
The  only  noteworthy  difference  is  that  the  sperm-cell  of  the 
fern,  like  the  sperm-cell  of  animals,  is  adapted  for  swimming 
in  fluids,  while  the  fertilizing  cell  derived  from  a  pollen-grain 
can  reach  the  egg-cell  in  a  pistil  only  through  a  pollen-tube. 
It  appears  that  the  development  of  pollen-tubes  in  the  seed- 
plants  is  an  adaptation  to  the  dry  conditions  surrounding  most 
flowers.  Even  water-lilies  and  other  aquatic  seed-plants 
have  stems  which  hold  the  flowers  above  the  water  so  that 
there  could  be  no  opportunity  for  a  swimming  cell  to  move 
from  an  anther  down  into  the  ovary.  The  dry  conditions  in 
all  flowers  of  seed-plants  are  well  adapted  to  the  transfer  of 
the  fertilizing  cell  or  sperm-cell  from  the  anther  to  the  egg- 
cell  by  means  of  (1)  pollination,  and  (2)  growth  of  a  pollen- 
tube  down  the  style  to  the  ovule. 

The  male  and  female  sex-cells  are  often  called  gametes, 
(meaning  bodies  which  unite  with  similar  bodies) ;  hence  in 
some  books  the  egg-cell  or  ovum  is  named  female  gamete, 
and  the  sperm-cell  is  the  male  gamete. 

Development  of  the  Fertilized  Egg-cell.  —  The  fertilized 
egg-cell  (also  called  gametospore,  or  oosperm,  meaning 
combined  egg  and  sperm)  of  the  fern,  begins  development 
soon  after  fertilization.  By  repeated  cell-division  similar  to 
that  already  described  (§  59),  the  egg-cell  is  divided  into  a 
large  number  of  cells,  which  forms  an  embryo,  and  this  grows 
into  a  plantlet  as  shown  in  Fig.  72,  G.  In  it  can  be  seen 
cotyledons,  stem,  and  root.  Soon  afterward,  the  prothallium 
withers,  and  the  young  fern  plant  becomes  independent.  It 
grows  to  maturity,  produces  spores  on  its  leaves,  and  these 
spores  in  turn  start  another  cycle  or  life-history. 

226.  Alternation  of  Generations  in  Ferns.  —  It  is  evident 
from  the  above  account  that  the  complete  life-history  of  a 
fern  is  composed  of  two  individuals  quite  different  in  appear- 
ance and  other  characteristics,  especially  in  the  method  of 
reproducing.  The  fern  plant  reproduces  by  means  of  spores 


STUDIES   OF  SPOKE-PLANTS  239 

formed  without  sex-organs ;  that  is,  by  asexual  reproduction. 
The  prothallium  reproduces  the  fern  plant  by  sex-cells 
(egg-cells  and  sperm-cells).  The  two  are  necessary  to  make 
a  complete  life-history,  for  the  spores  cannot  develop  directly 
into  a  fern  plant  and  the  fertilized  egg-cell  cannot  develop 
directly  into  a  new  prothallium.  The  prothallium  and  the 
fern  plant  each  represents  what  is  known  in  biology  as  a 
generation,  and  the  succeeding  each  other  is  alternation  of 
generations.  It  should  be  especially  noted  that  a  generation 
reproducing  asexually  (i.e.,  the  fern  plant)  alternates  with 
the  one  which  reproduces  by  the  sexual  method  (i.e.,  the 
prothallium) . 

The  asexual  generation  represented  by  the  fern-plant  is 
often  known  as  the  sporophyte  (meaning  spore-plant,  because 
it  forms  spores) ;  while  the  sexual  generation,  the  prothal- 
lium, is  known  as  gametophyte  (meaning  gamete-plant, 
because  it  develops  the  gametes  or  male  and  female  sex-cells). 

It  is  interesting  to  note  that  the  spore-generation,  the  fern 
plant,  may  live  very  many  years  and  reach  a  large  size  (some 
tree-ferns  of  the  tropics  are  forty  or  more  feet  high).  The 
sex-generation,  the  prothallium,  is  always  very  small  and  lives 
only  a  few  months.  In  fact,  it  is  so  small  and  attracts  so 
little  attention  that  only  by  most  careful  studies  were  the 
facts  concerning  alternation  of  two  generations  discovered. 

By  life-history,  or  life-cycle,  of  a  fern  we  understand  all 
stages  from  spore  to  spore  again ;  that  is,  spore,  prothallium, 
sex-cells,  fertilized  egg-cell,  embryo,  fern-plant  which  forms 
spores  starting  a  new  life-history. 

227.  Alternation  of  Generations  in  Seed-plants.  —  The 
similarity  of  structure  between  roots,  stems,  and  leaves  of 
ferns  and  the  lowest  seed-plants  suggests  that  they  are 
related.  The  fact  that  the  ferns  have  alternation  of  two 
generations  (prothallium  and  fern  plant)  makes  their  life- 
history  appear  at  first  to  be  entirely  different  from  seed- 
plants;  but  careful  investigations  have  shown  that  even 


240  APPLIED  BIOLOGY 

seed-plants  have  two  generations  alternating  in  their  life- 
history.  One  of  these  generations  (the  sporophyte  or  spore- 
forming  stage)  is  the  ordinary  seed-plant  (e.g.,  an  oak  tree) ; 
and  its  spores  are  pollen-grains  and  ovules.  The  oak  tree 
then  corresponds  to  the  fern-plant  in  that  each  is  the  spore- 
producing  stage.  The  other  generation  (the  gametophyte, 
which  produces  the  sex-cells)  in  seed-plants  is  the  pollen- 
tube,  in  which  is  formed  the  sperm-cell,  and  a  microscopic 
structure  (the  embryo-sac)  in  the  ovule,  in  which  is  formed 
the  egg-cell.  The  development  of  the  sperm-cell  in  the  pollen- 
tube  and  of  the  egg-cell  in  the  embryo-sac  of  seed-plants  is 
believed  to  be  a  great  modification  of  the  formation  of  sperm- 
cells  and  egg-cells  in  the  sex-organs  of  prothallia  of  the  lower 
plants.  In  ferns  the  spores  are  all  alike,  but  in  seed-plants 
there  are  two  kinds,  —  the  smaller  microspores  or  pollen- 
grains,  and  the  larger  megaspores  or  ovules.  A  minor  differ- 
ence is  that  the  spores  of  ferns  grow  extensively,  forming 
prothallia,  which  later  produce  sex-cells,  but  seed-plants 
develop  the  sex-cells  by  cell-divisions  inside  the  spores.  Thus 
in  both  ferns  and  seed-plants  the  sex-cells  come  more  or  less  • 
indirectly  from  spores,  and  the  united  sex-cells  of  the  two  kinds 
develop  into  ferns  and  seed-plants,  which  in  their  turn  form 
spores  again. 

228.  Allies  of  Ferns.  —  In  addition  to  the  plants  which 
are  easily  recognized  as  ferns,  and  of  which  there  are  numerous 
species  in  the  United  States,  there  are  a  number  of  closely 
related  fern-like  plants.  These  are  the  water-ferns,  the  horse- 
tails (Equisetum),  and  the  club-mosses  (lycopods).  Speci- 
mens (fresh  or  in  formalin)  should  be  exhibited  in  order  to 
give  general  acquaintance.  If  time  permits,  special  text- 
books of  botany  should  be  consulted  as  guides  to  very  brief 
study  of  specimens  of  these  plants. 

All  the  fern-like  plants  taken  together  constitute  a  group 
known  as  the  Pteridophytes  (Pteridophyta),  one  of  the  pri- 
mary divisions  of  the  plant  kingdom  (see  table  in  Chapter 


STUDIES   OF  SPORE-PLANTS  241 

VII).  The  group  was  much  more  important  in  the  early 
periods  of  the  earth's  history  than  at  present.  During  the 
Carboniferous  period,  pteridophytes  formed  the  main  mass 
of  the  land  vegetation ;  but  they  decreased  in  prominence 
in  the  later  period  when  the  gymnosperms  and  still  later 
the  angiosperms  appeared.  The  horse-tails  and  club-mosses 
which  exist  to-day  are  rather  small  plants,  but  some  of  the 
ancient  species  which  lived  before  the  end  of  the  Carbon- 
iferous period  were  stately  trees. 

229.  Economic  Relations   of  Pteridophytes.  —  The  ferns 
and  their  allies  furnished  much  of  the  organic  material  which 
was  transformed  into  coal.     Many  leaves  and  stems  of  ferns 
and  allied  plants  are  found  as  coal  fossils,  splendid  specimens 
of  which  are  on  exhibition  at  natural  history  museums. 

At  the  present  time  little  use  is  made  of  these  plants  except 
for  ornamental  purposes,  for  which  their  foliage  is  unsur- 
passed. Many  tropical  species  of  ferns  are  kept  in  green- 
houses, and  several  species  have  long  been  favorite  house- 
plants.  Various  kinds  of  club-mosses  are  also  used  in  the 
same  way. 

One  fern  contains  in  its  stem  a  powerful  drug,  known  in 
pharmacy  as  "  extract  of  male  fern,"  and  often  used  for  ex- 
pelling tape-worms  and  round-worms  from  the  intestines. 

The  horse-tails  were  known  as  "  scouring  rushes  "  in  the 
pioneer  days  in  America,  because  their  stems  were  used  to 
scour  pewter  and  brass  utensils.  The  scouring  property  de- 
pends upon  the  silicon  particles  in  the  stems  (easily  dem- 
onstrated by  burning). 

MOSSES 

230.  Distribution  of  Mosses.  —  One  of  the  most  interesting 
points  concerning  mosses  is  their  wide  distribution,  which  is 
made  possible  by  adaptations  to  various  conditions  of  life. 
They  are  common  everywhere.     We  may  find  them  on  ex- 


242  APPLIED  BIOLOGY 

posed  and  barren  hillsides,  in  bogs,  in  cold  northern  regions, 
in  forests,  and  even  in  water. 

One  of  the  most  important  is  the  bog-  or  peat-moss 
(Sphagnum)  which  grows  luxuriantly  where  most  other  plants 
cannot  live,  and  its  growth  gradually  fills  the  bogs  with  dead 
vegetation  called  peat.  This  does  not  decay  rapidly  because 
the  water  in  peat-bogs  is  somewhat  antiseptic  (i.e.,  prevents 
the  development  of  bacteria  which  cause  decay  elsewhere). 
In  Ireland  and  some  other  countries  the  peat  which  is  ex- 
tensively used  as  fuel  is  composed  largely  of  species  of  peat- 
moss or  sphagnum-moss.  The  American  peat  contains  many 
other  aquatic  plants.  The  floating  islands  in  some  lakes 
and  the  quaking  soil  in  marshy  regions  are  masses  of  dead 
vegetation,  often  largely  peat-moss,  which  have  collected 
some  silt  and  thus  formed  soil  on  which  other  kinds  of  plants 
grow.  Sphagnum  moss  is  extensively  used  for  packing 
plants  for  shipping;  and  in  greenhouses  for  filling  hanging 
baskets  for  ferns  and  orchids,  for  mixing  with  soil  to  prevent 
it  from  packing  and  to  make  it  hold  water,  and  for  covering 
the  soil  in  flower-pots. 

231.  Structure  of  a  Moss.  —  Specimens  of  several  common 
mosses  should  be  collected  for  comparison  in  connection  with 
the  following  account  made  brief  by  limitation  of  the  time 
which  can  be  allowed  for  study  of  mosses  in  a  year's  course  of 
biology.  Specimens  of  common  large  mosses  should  be 
collected  in  summer  and  preserved  in  formalin-solution. 

Most  of  our  common  mosses  have  erect  stems  with  leaves 
arranged  radially.  Sometimes  the  older  part  of  the  stem 
is  prostrate.  A  section  shows  no  wood-tubes,  as  in  higher 
plants  (ferns  and  seed-plants),  but  only  a  central  axis  of  cells 
which  serve  to  conduct  fluids.  At  the  base  of  the  stem  are 
rootlets  or  rhizoids,  often  twisted  together.  The  leaves  have 
a  very  simple  mid-vein  of  elongated  cells,  but  no  such  com- 
plex veining  as  in  higher  plants.  Some  mosses  growing  in  dry 
places  (e.g.,  Polytrichum)  have  one  side  of  the  leaves  specially 


STUDIES   OF  SPORE-PLANTS  243 

adapted  to  the  light-relation,  and  this  delicate  side  is  protected 
during  drought  by  rolling  of  the  edges  of  the  leaf. 

232.  Reproduction  of  Mosses.  —  Many  creeping  branches 
form  rootlets,  and  after  decay  of  the  older  stem  may  become 
independent.  This  is  the  chief  cause  of  the  mats  of  mosses 
often  found.  But  in  addition  to  this  vegetative  propagation, 
all  mosses  reproduce  by  means  of  sex-cells  and  by  spores. 
Sex-organs  (ovaries  and  spermaries)  are  formed  in  the  bud- 
like  apex  of  the  stem  or  of  the  main  branches.  Both  kinds  of 
sex-organs  are  in  some  species  found  on  the  same  plant  (i.e., 
it  is  monoecious) ;  but  in  other  species  on  separate  plants 
(dioecious}.  A  spermary  (botanists  usually  call  it  an  an- 
theridium  in  mosses  and  ferns)  is  an  elongated  club-shaped 
mass  of  cells  (Fig.  73,  E),  the  inner  ones  of  which  become  con- 
verted into  swimming  sperm-cells,  which  are  freed  by  rupture 
of  the  wall  of  the  spermary.  An  ovary  (usually  called  arche- 
gonium,  as  in  ferns)  has  the  shape  of  a  chemist's  flask  with 
a  long  tubular  neck  (Fig.  73,  F).  In  the  rounded  base  of  the 
ovary  is  an  egg-cell  or  ovum.  This  is  fertilized  by  a  sperm- 
cell  which  swims  down  the  neck  of  the  ovary.  The  fertilized 
egg-cell  (or  oosperm)  divides  into  two,  four,  eight,  and  more 
cells,  forming  the  embryo.  This  soon  begins  to  grow  upward 
as  a  slender  rod,  and  the  end  of  the  rod  expands  into  and 
forms  a  vase-like  structure  (Fig.  73,  C).  This  is  a  spore-case, 
usually  called  by  botanists  the  sporangium,  and  its  inner 
ceils  form  numerous  spores.  It  usually  has  a  cap-like  lid, 
which  falls  off  when  the  spores  are  ripe  and  ready  for  scat- 
tering. The  entire  stalk  and  spore-case  formed  from  the 
developing  egg-cell  is  called  sporogonium. 

When  a  moss  spore  begins  to  germinate,  a  delicate  thread  or 
filament  grows  out  and  branches.  This  structure  formed 
by  the  germinating  spore  is  called  a  protonema  (meaning  the 
first  thread). 

After  a  time  one  or  more  buds  appear  on  the  sides  of  this 
protonema,  and  from  them  moss  plants  grow  upward.  Thus 


244 


APPLIED  BIOLOGY 


a  spore  may  form  one  or  more  moss  plants.  The  protonema, 
then,  is  a  sort  of  creeping  stem  able  to  produce  erect  branches. 
It  is  part  of  the  moss  plant. 


FIG.  73.  Moss.  A,  B,  plant  with  spore-case  (&)  which  has  a  lid  (c) ;  z,  root 
hairs.  C,  enlarged  view  of  spore-case.  D,  sperm-cell.  E,  spermary, 
sperms  escaping.  F,  ovary  with  embryo  (o).  G,  protonema;  b,  bud 
from  which  moss  plant  develops.  H,  spore  (s)  germinating  to  form  a 
protonema.  (After  Parker,  Strasburger,  and  others.) 

233.  Alternation  of  Generations  of  Moss.  —  It  is  evident 
that,  as  in  the  ferns,  the  moss  has  an  alternation  of  genera- 
tions. The  moss  plant  is  the  stage  or  generation  which  bears 
the  sex-organs,  while  the  sporogonium  produces  the  asexual 


STUDIES  OF  SPORE-PLANTS  245 

spores  which  germinate  to  produce  a  new  moss  plant. 
Obviously,  the  moss  plant  corresponds  to  the  fern  prothal- 
lium,  because  each  is  produced  from  a  spore  and  each  is  a 
gametophyte  and  produces  sex-organs.  Moreover,  the  moss 
sporogonium  and  the  fern  plant  both  develop  from  fertilized 
egg-cells,  and  produce  asexual  spores.  This  relation  is  ex- 
pressed in  diagram  form  as  follows,  the  dashes  indicating  the 
order  of  stages. 

Fern  spore  —  pro  thallium  (gametophyte)  —  sex-cells  —  fern  plant 
(sporophyte)  —  spores. 

Moss  spore  —  moss  plant  (gametophyte)  —  sex  cells  —  sporo- 
gonium (sporophyte)  —  spores. 

The  striking  difference  is  that  the  spore-forming  stage 
(sporophyte)  is  the  short-lived  and  relatively  inconspicuous 
sporogonium  of  the  moss  and  the  prominent  fern  plant.  On 
the  other  hand,  the  sex-generation  (gametophyte)  of  the  fern 
is  the  prothallium  which  few  people  ever  see,  and  in  the  moss 
it  is  the  well-known  plant. 

234.  Allies   of    Mosses :    Bryophytes.  —  The    only    close 
relatives  of  mosses  are  the  liverworts,  of  which  Marchantia  is 
an  example  common  in  greenhouses.     The  mosses  and  liver- 
worts together  constitute  a  group  known  as  Bryophytes  (mean- 
ing moss-plants).     Certain  lichens  (§  248)  and  other  plants 
are  sometimes  mistaken  for  true  mosses.     See  relation  of 
mosses  to  other  groups  of  plants  in  table  of  classification  in 
§133. 

H.    LOWER  SPORE  PLANTS* 

235.  Algae  and  Fungi.  —  So  far  all  plants  studied  (seed- 
plants,  ferns,  mosses)  have  more  or  less  similarity.     Even 
mosses  have  the  working  equivalent  of  the  roots,  stem,  and 
leaves  as  seen  in  the  higher  seed-plants.     We  shall  next  study 
some  types  of  still  lower  and  simpler  plants,  which  in  fact 


*  The  higher  spore-plants  are  the  ferns  and  mosses,  described  in  the  fore- 
going sections. 


246  APPLIED  BIOLOGY 

are  so  simple  that  there  is  no  differentiation  of  the  plant- 
body  into  roots,  stem,  and  leaves.  Some  of  these  lower  plants 
have  chlorophyll  and  are  called  Algae ;  and  some  (the  Fungi) 
have  no  chlorophyll.  All  these  simple  plants  belong  to  the 
lowest  division  of  the  Thallophytes.  See  table  of  plant 
classification  in  §  133. 

1.   LOWER  SPORE-PLANTS  WITH  CHLOROPHYLL :  ALG^J 

236.  The  lowest  spore-plants  without  root,  stem,  and 
leaves  and  with  chlorophyll  are  often  grouped  together 
under  the  name  Algae.*  These  are  numerous  forms,  rang- 
ing from  microscopic  plants  to  the  large  sea-weeds.  Most 
of  the  algae  live  in  fresh  or  salt  water,  but  a  few  can  live  in 
moist  places  on  soil  and  surfaces  of  larger  plants,  and  even 
occasionally  undergo  drying. 

In  this  course  we  cannot  take  time  for  more  than  a  brief 
study  of  a  few  common  types  whose  life-activities  help  us 
to  a  better  understanding  of  some  of  the  general  principles 
of  biology.  But  the  reader  should  understand  that  such 
a  condensed  account  as  that  which  follows  cannot  be  more 
than  a  beginning  study  of  a  few  examples  in  a  great  group 
of  low  plants  upon  which  much  emphasis  is  laid  in  advanced 
courses  of  botany  in  colleges.  In  the  present  study  we  can 
do  little  more  than  direct  attention  to  the  ways  in  which 
these  simple  plants  solve  the  problems  of  the  fundamental 
life-activities  of  feeding,  moving,  breathing,  excreting,  and 
reproducing  —  processes  which  we  have  now  traced  through 
a  series  of  plants,  beginning  with  the  highest  or  seed-plants. 

The  following  descriptions  of  common  types  of  algae  should 
be  accompanied  as  far  as  possible  by  demonstrations  and 
laboratory  study. 


*  A  few  of  the  highest  algae  (e.g.,  the  freshwater  Chara  and  Nitella)  have 
structures  serving  as  roots,  stems,  and  leaves  very  much  as  might  a  moss 
plant  entirely  submerged  in  water. 


STUDIES  OF  SPORE-PLANTS 


247 


237.  Pleurococcus.  —  A  green  coating  or  stain  which  is 
often  seen  on  the  shaded  (usually  north)  side  of  tree-trunks, 
unpainted  walls,  etc.,  will  be  found  upon  microscopic  ex- 
amination to  be  composed  of  masses  of  one-celled  plants, 
often  arranged  in  groups  of  two,  four,  or  more  cells  (Fig.  74). 
This  grouping  is  the  result  of  repeated  cell-division.     Each 
cell  has  protoplasm,  nucleus,  chlorophyll-bodies,  and  cell- 
wall.     Having  chlorophyll,  they 

are  able  to  make  carbohydrates 
from  carbon  dioxide  and  water. 
Particles  of  dust  lodged  on  the 
wood  on  which  they  grow  prob- 
ably supply  the  necessary  ma- 
terials which  higher  plants 
absorb  from  the  soil.  Abun- 
dance of  water  favors  growth, 
and  they  appear  a  brighter 
green  during  rainy  weather. 

238.  Sphaerella.  —  Red  stains 
are   often   seen    in    hollows    in 
rocks,  in  empty  urns  in  ceme- 
teries,   in  roof-gutters,    and  in 

similar  places  where  dust  may  collect  and  where  temporary 
pools  of  water  may  be  formed  by  rains.  If  water  is  present 
in  such  hollows,  it  may  be  reddish  or  greenish  in  color. 
Collect  some  of  the  dry  dust,  decayed  leaves,  etc.,  from  such 
places  during  the  autumn,  and  keep  dry  until  wanted  for 
study.  Then  place  some  of  the  dry  material  in  water,  and 
expose  to  light  for  several  days.  Examine  with  the  micro- 
scope some  of  the  dry  material  mounted  in  a  drop  of  water 
and  observe  spherical  cells  with  red  contents  (protoplasm 
plus  a  coloring  matter),  and  with  thick  cell-walls.  This  is 
called  the  "  resting  stage  "  of  the  Sphaerella  plant.  By  special 
methods  it  is  possible  to  extract  the  color,  and  demonstrate 
a  nucleus  near  the  center.  It  is  evidently  a  one-celled  plant. 


FIG.  74.  Pleurococcus.  a,  one  cell; 
6,  two  cells  formed  by  division  of 
a;  c,  three  cells  by  division  of  one 
of  two  cells  in  b;  d,  four  cells; 
cw,  cell-wall;  n,  nucleus;  cell- 
body  between  nucleus  and  cell- 
wall  contains  chlorophyll.  (From 
Sedgwick  and  Wilson.) 


248  APPLIED  BIOLOGY 

Placed  in  water  and  light  the  contents  divide  into  four,  eight, 
or  sixteen  masses,  the  cell-wall  breaks,  and  each  one  of  these 
masses  becomes  a  motile  stage,  having  the  appearance  shown  in 
Fig.  75,  A.  The  two  whip-like  structures  (flagella)  have 
lashing  movements  which  propel  the  organism.  The  color  in 
this  stage  may  be  red,  green,  or  a  mixture  of  the  two  colors. 
Carbohydrates  are  made  from  carbon  dioxide  absorbed  from 
the  water.  The  organism  also  absorbs  nitrogen  compounds 

and  other  necessary  elements  from 
the  water,  forms  new  protoplasm, 
and  soon  grows  to  full  size.     After 
a  time  the  flagella  are  retracted; 
f       the  plant  enters  the  resting  stage ; 
FIG.  75.  Sphserella.  ^.motile     and  when  conditions  are  favorable, 
stage;  /  flagella     B  four     tne  protoplasmic  contents  divide 

like  A  inside  cell-wall,  formed  .       .    ,        /• 

by  division  of  a  non-motile     again  into  four  or  more  masses, 
form  similar  to  Fig.  74  a.   c,     which  form  the  swimming  stage. 

division    into    numerous  mi  •    j.          j.* 

small  cells  like  D.  E,  con-         There  are  numerous  interesting 
jugation  of  two  like  D,  form-     details  of  structure  and  life-history, 

(FromStras-      but  thege  must  be  jeft   f()r  special 

courses  in  botany.  For  our  present 
purposes  the  important  facts  are  that  Sphserella  is  (1)  a  one- 
celled  (unicellular)  plant ;  (2)  it  reproduces  by  a  process  of 
cell-division;  (3)  it  is  able  to  make  its  own  carbohydrate 
foods  from  carbon  dioxide  and  water  by  the  action  of  light 
on  the  protoplasm'containing  coloring  matter  which  acts  like 
chlorophyll;  and  (4),  most  remarkable  of  all,  it  swims  in  an 
animal  fashion  in  one  stage  of  its  existence. 

Plant  or  Animal  ? — We  classify  Sphserella  as  a  plant  be- 
cause its  nutrition  is  the  same  as  that  of  higher  green  plants. 
No  known  animal  can  make  carbohydrate  foods  from  car- 
bon dioxide  and  water.  The  one  thing  about  Sphserella 
which  suggests  that  it  is  an  animal  is  that  it  swims ;  but  we 
have  already  noted  that  reproductive  cells  of  ferns  and  mosses 
can  swim,  and  that  many  higher  plants  have  movements. 


STUDIES  OF  SPORE-PLANTS 


249 


Sphaerella  is  in  some  books  called  Haematococcus  (mean- 
ing blood-berry).  This  is  only  one  of  many  examples  of 
plants  which  have  two  names.  The  explanation  is  that  it  is 
one  of  the  rules  of  the  botanical  societies  that  the  name 
first  given  by  a  scientific  man  who  accurately  describes  a  new- 
found species  should  be  the  accepted  one.  In  the  case  of  the 
plant  under  discussion,  there  is  still  some  doubt  regarding 
the  first  description ;  and  hence  there  must 
be  further  investigations  before  it  can  be 
decided  that  one  of  the  names  is  to  be  kept 
in  the  scientific  books  of  the  future  and  the 
other  allowed  to  become  obsolete.  There 
are  many  other  such  undecided  problems 
concerning  the  naming  of  other  plants,  and 
also  of  animals. 

239.  Spirogyra.  —  In  many  ponds  and 
streams  there  are  floating  green  masses  of 
delicate  thread-like  plants  popularly  known 
as  pond-scum.  These  threads  are  com- 
posed of  a  row  of  elongated  cylindrical  cells 
(Fig.  76),  with  their  chlorophyll  arranged 
in  one  or  more  spiral  bands.  A  large  nu- 
cleus may  be  seen  near  the  center  of  each 
cell.  The  cells  grow  and  divide  so  as  to 
form  long  filaments  composed  of  chains  of 
cells.  Each  cell  is  an  individual  plant,  because  it  is  capable 
of  carrying  on  all  the  life-processes  of  a  green  plant. 

Under  certain  conditions  a  tube  may  grow  so  as  to  join 
two  adjacent  cells  of  the  same  or  of  separate  filaments,  and 
through  this  tube  the  protoplasm  of  one  of  the  two  cells  passes 
into  .the  other  and  the  united  substance  forms  a  spore 
(zygospore).  This  later  germinates  and  forms  a  cell  which 
by  repeated  growth  and  division  forms  a  filament  or  chain 
of  cells.  This  process  leading  to  spore-forming  is  known 
as  conjugation.  A  similar  process  occurs  in  the  black  mold 


FIG.  76.  A  cell  from 
a  filament  of  Spi- 
rogyra. A;,  nucleus 
in  center;  two 
spiral  bands  con- 
taining chlorophyll 
(ch). 


250 


APPLIED  BIOLOGY 


(§  244).     Like  fertilization  in  higher  plants,  this  conjugation 
mixes  protoplasm  from  two  individual  Spirogyra  plants,  and 
the  mixture  appears  to  have  increased  vitality  (Fig.  77). 
The  physiology  of  such  a  one-celled  green  plant  living  in 
water  is  briefly  as  follows  :  It  absorbs  water 
and  carbon  dioxide,  and  these  are  used  in 
the  making  of  starch  or  other  carbohydrate 
foods.     From  water  also  the  plants  absorb 
the  other  elements  necessary  (N,  S,  P,  K, 
etc.)  for  making  new  protoplasm.     These 
elements  are  usually  the  same  as  in  the 
higher  green  plants.     Also,  the  plant  ab- 
sorbs from  the  water  the  oxygen  required 
FIG  77    A  conjuga-  ^or  ^e  life-activities.     The  carbon  dioxide 
tion  between  cells   produced  by  oxidation  of  cell-substances  is 
of  two  adjacent   abSOrbed  by  the  surrounding  water;    but 

filaments    of    Spi-  .  .        '  ' 

rogyra.     Concen-   during  exposure  to  light  is  probably  used 
tration  of  proto-   m  making  carbohydrates  by  photosynthe- 
sis.    Probably    other    excretions    besides 
carbon  dioxide  are  formed  in  the  one-celled 
plants  and  absorbed  by  the  surrounding 

progress    in    next    water. 

240.  Other  green  algae  are  numerous, 
is  beginning  in  low-  They  appear  as  greenish  coatings  on  various 
est  pair  of  cells,  objects  in  water,  on  the  bottom  of  springs 

B,  conjugation  be-          J  . 

tween   contiguous  and  ponds,  and  on  pots  and  soil  in  green- 
ceils  of  the  same  houses.     Certain  species  are  likely  to  grow 
on  the  glass  of  aquaria.     In  such  a  position 
it  is  easy  to  show  their  dependence  upon  light  by  shading 
the  glass  with  black  paper  in  which  openings  (possibly  in  the 
form  of  letters)  have  been  cut.     The  alga3  will  grow  well 
only  where  they  receive  the  light.     For  descriptions  of  the 
numerous  forms  which  are  of  common  occurrence,  the  student 
must  refer  to  textbooks  specially  devoted  to  the  lower  green 
plants. 


plasm  into  one  cell 
of  each  pair,  form- 
ing a  zygospore  (z) , 


STUDIES  OF  SPORE-PLANTS  251 

241.  Brown  and   Red  Algae.  —  These   are  chiefly  marine 
plants,  and  are  commonly  known  as  sea-weeds.    Numerous 
kinds  of  these  may  be  seen  on  any  sea-shore.     The  red  and 
brown  colors  appear  to  be  due   to   pigments  which  conceal 
the  chlorophyll.     At  any  rate  these  plants  make  their  own 
carbohydrate  foods  from  carbon  dioxide  and  water  just  as 
higher  green  plants  do.     Studies  of  the  structure  and  life- 
histories  of   these  interesting  algae  must  be  left  for  college 
courses  of  botany. 

242.  Economic  Relations  of  Algae.  —  Probably  the  great- 
est value  of  the  algae  lies  in  the  fact  that  they  make  food  for 
higher  organisms.     This  may  be  observed  in  any  aquarium 
where  snails  rasp  off   and  eat  the  green  algae  on  the  glass. 
Numerous  small  animals  eat  algae,  and   these  animals  may 
serve  as  food  for  still  larger  animals.     Immense  quantities  of 
these  low  plants  are  found  in  the  ocean  down  as  far  as  light 
penetrates.     Even  some  large  animals,   e.g.,   certain  fishes 
and  whales,  have  strainers  in  their  mouths  which  enable  them 
to  collect  large  quantities  of  very  small  organisms,  some  of 
them  animals  and  some  plants ;  but  especially  do  small  ani- 
mals which  feed  on  simple  algae  serve  as  food  for  the  larger 
animals.     It  is  now  quite  certain  that  directly  or  indirectly 
the  simplest  green  plants  play  an  important  part  in  the  food- 
supply  of  aquatic  animal  life,  much  of  which  is  of  use  to  man. 
In  this  line  the  simple  algae  have  great  economic  importance. 

Many  kinds  of  the  larger  sea-weeds  have  some  economic 
uses.  Thus  iodine  is  prepared  from  the  ash  obtained  by 
burning  sea-weeds,  and  formerly  this  was  also  the  chief  source 
of  sodium  carbonate,  from  which  baking  soda  is  made.  Cer- 
tain brown  sea-weeds  are  used  as  food  by  Chinese  and 
Japanese  and  by  the  natives  of  the  Malay  Archipelago. 
Agar-agar  and  Irish  moss,  both  used  in  preparation  of  jellies, 
are  made  from  certain  kinds  of  sea-weeds.  Finally,  in  some 
agricultural  countries  near  the  sea  the  sea-weeds  cast  up  by 
the  waves  are  used  as  soil  fertilizers. 


252  APPLIED  BIOLOGY 

Algce  in  Water  Reservoirs. — On  the  harmful  side  there  is 
very  little  to  be  said  against  the  plants  of  the  algae  group. 
Probably  most  important  is  the  fact  that  certain  species 
grow  extensively  in  reservoirs  used  for  storing  water,  and 
impart  a  very  disagreeable  odor  and  flavor  to  the  water. 
Some  of  the  cities  in  eastern  Massachusetts  and  New  York 
City  have  had  much  trouble  in  this  line.  A  small  amount  of 
copper  sulphate  dissolved  in  the  water  prevents  the  growth 
of  such  algae ;  but  such  chemicals  must  be  used  with  caution 
until  scientific  experiments  demonstrate  whether  or  not 
small  quantities  in  drinking  water  may  not  be  harmful 
when  used  continually. 

2.   SPORE-PLANTS  WITHOUT  CHLOROPHYLL:  FUNGI 

243.  The  most  important  examples  of  spore-plants  which 
have  no  chlorophyll,  and  hence  must  be  in  their  food-sup- 
ply different  from  green  plants,  are  common  molds,  mush- 
rooms,  and  yeast-plants.     To  these  might  be  added  the 
bacteria ;  but  many  peculiarities  and  their  great  importance 
make  it  best  to  devote  a  separate  section  to  them  (beginning 
with  §  254). 

Molds 

244.  Study  of  Common  Molds.  —  It  is  a  well-known  fact 
that  when  bread,  cheese,  or  other  foods  are  left  for  some  time 
in  a  damp  place,  there  appears  over  the  surface  a  mass  of 
delicate  threads  or  filaments,  and  the  bread  is  said  to  be 
"  molding "  or  "  moldy."     Soon   the   color  of  the  moldy 
mass  changes   from  white  to  black,  blue-green,  or  brown, 
and  according  to  the  color,  the  names  black  mold,  green 
mold,  and  brown  mold  are  applied. 

(L)  Examine  pieces  of  bread  on  which  black  mold  is  growing. 
Break  off  pieces  and  note  that  delicate  threads  run  through  the 
bread  (use  a  hand-lens).  These  threads  are  the  hyphce,  and  the 
entire  mass  of  them  is  the  mycelium.  Starting  on  the  surface,  the 


STUDIES  OF  SPORE-PLANTS 


253 


mycelium  will  grow  in  all 
directions  until  an  entire 
loaf  of  bread  is  interlaced 
with  the  hyphae.  At  the 
same  time  the  hyphae  secrete 
digestive  substances  which 
digest  to  a  soluble  form 
starch  and  other  foods  in 
the  bread.  These  digested 
or  dissolved  foods  are  ab- 
sorbed by  the  hyphse  and 
used  for  growth.  The  mold 
is  therefore  a  saprophyte 
(§98). 

The  delicate  threads  seen 
above  the  surface  of  the 
bread  are  called  erect  or 
aerial  hyphas;  they  are 
branches  of  the  mycelium. 


FIG.  79.     Black  mold, 
aerial  hyphse   (a.hy), 


(spg).  B,  immature  spore-case.  C,  mature.  D, 
liberating  spores  (sp).  F,  spores.  G,  H,  germi- 
nation of  spores  to  form  new  mycelium.  (From 
Parker.) 


FIG.  78.  A  black  mold  plant  formed  from 
one  spore  in  center.  One  mature  aerial 
hypha  with  spherical  spore-case  (s),  and 
two  immature  ones.  Root-like  structure 
is  mycelium.  (From  Rny.) 

Close  examination  with  a  strong  hand- 
lens  will  show  that 
the  color  of  the  mold 
is  due  to  bodies  at- 
tached to  the  ends, 
of  the  erect  hyphas, 
These  bodies  are  the 
spore-cases,  or  spo- 
rangia, which  are 
filled  with  spores. 
The  purpose  of  the 
aerial  hyphae  ap- 
pears to  be  produc- 
tion of  the  spore- 
cases  and  elevation 
of  them  so  that  wind 
currents  may  better 
distribute  the  spores. 
It  is  easy,  with  a 
low-power  micro- 
scope, to  find  various 
stages  in  the  de- 
velopment of  the 
spore-cases  which 


A,  mycelium  (my)  with  two 
each  forming   a   spore-case 


254 


APPLIED  BIOLOGY 


are  formed  by  expansion  of  the  ends  of  aerial  hyphse.  The  proto- 
plasm inside  these  expanded  ends  divides  into  numerous  small 
masses,  each  of  which  becomes  a  spore.  Examine  with  high  power 
of  microscope  spores  obtained  by  brushing  a  wet  needle  or  small  brush 

against  some  sporangia  and 
then  rubbing  the  spores  col- 
lected into  a  drop  of  water 
on  a  glass  object-slide. 
Notice  the  thick  cell-wall, 
which  is  very  protective 
against  heat,  drought,  and 
other  unfavorable  con- 
ditions. 

(D)    If   some   spores   be 
placed  in  a  dilute  sugar  solu- 


FIG.  80.     Unlike  the  spherical  spore-case  of 
the  black  mold,   the  green  mold  forms 


tion  in  a  watch-glass,  many 

spores  (sp)  at -ends Tof  branches i  of  aerial    of  them  will  be  found  ger- 
hyphae.  minating  after  a  day  ;  and 

after    several    days    much 

branching  will  produce  an  extensive  mycelium  (Fig.  79).  Later  some 
aerial  branches  or  hyphae  arise  and  form  spore-cases.  The  entire 
mycelium  and  aerial  branches  with  spore-cases,  which  originate  from 
a  single  spore,  is  a  mold  plant.  Usually  such  plants  are  closely  in- 
terlaced, because  many  spores  fall  on  the  same  piece  of  bread. 


Sometimes  a 
peculiar  method 
of  reproduction 
occurs  as  follows: 
From  each  of  two 
hyphae  which 
happen  to  lie 
near  together, 
c  1  u  b-s  h  a  p  e  d 
bodies  grow  until 
their  ends  meet 
(Fig.  81).  The  protoplasmic  contents  in  the  ends  of  the 
two  bodies  unite  into  one  mass,  and  around  it  a  thick  wall 
or  coat  forms.  There  is  thus  formed  a  combined  spore 


b  c  d  e 

Stages  in  conjugation  of  two  mold  hyphae, 


a 

FIG.  81 

forming  the  spore  (zyg). 


(From  Parker.) 


STUDIES  OF  SPORE-PLANTS  255 

(called  zygospore,  or  gametospore),  which  like  the  fertilized 
egg-cells  of  mosses,  ferns,  seed-plants,  and  animals,  is  com- 
posed of  protoplasm  from  two  sources.  In  short,  this 
zygospore  is  a  simple  kind  of  fertilized  egg-cell.  It  germi- 
nates much  like  an  ordinary  mold  spore,  and  forms  hyphae 
which  develop  aerial  branches  with  ordinary  spore-cases. 
Apparently  the  reason  for  this  process,  known  as  conjugation 
of  molds,  is  the  same  as  that  for  fertilization  in  other  plants 
and  in  animals;  namely,  that  there  is  some  physiological 
advantage  in  new  individual  organisms  beginning  from  a 
mass  of  protoplasm  which  has  two  parents.  The  significance 
of  this  is  in  part  a  problem  of  heredity,  which  will  be  pre- 
sented in  the  last  chapter. 

As  a  rule,  molds  grow  best  in  a  warm  place,  as  can  be 
proved  by  leaving  pieces  of  moist  bread  in  two  bottles, 
one  of  which  is  placed  near  a  stove  or  radiator,  and  one  in 
an  ice-box.  However,  some  molds  will  grow  on  foods  kept 
for  some  time  in  the  ice-box,  and  one  kind  forms  the  slimy 
material  which  grows  in  the  drain-pipes  of  ice-boxes  and 
clogs  them  unless  cleaned  frequently  by  hot  water  and  wash- 
ing powder. 

245.  Experiments  with  Molds.  —  (D  or  L)  Test-tubes  £  by  5 
inches  are  best  for  these  -experiments,  but  bottles  with  mouth  at 
least  \  inch  in  diameter  will  do.  Cut  some  strips  of  bread  i  by  2  inches 
in  size,  and  place  one  in  each  of  ten  or  more  tubes,  and  add  three  or 
four  drops  of  boiled  water  to  each  tube.  Now,  plug  the  mouths  of 
the  tubes  with  cotton-batting  or  absorbent  cotton,  making  the  plugs 
by  rolling  the  cotton  into  a  cylinder  about  two  inches  long.  Have 
the  plugs  of  a  size  which  will  fit  rather  loosely  in  the  tubes,  else 
after  steaming  the  cotton  will  swell  and  the  plugs  will  be  forced 
out.  Fold  the  projecting  cotton  over  the  edge  of  the  tube,  so  as 
to  keep  dust  from  falling  on  the  edge.  Experiments  tried  thousands 
of  times  have  shown  that  spores  and  microscopic  organisms  cannot 
get  through  such  plugs  of  dry  cotton,  and  that  the  tubes  are  as 
effectually  closed  as  if  the/  were  hermetically  sealed  by  melting  the 
glass.  Also,  the  cotton  has  the  advantage  of  allowing  the  entrance 
of  air,  which  is  needed  by  molds  that  grow  in  the  tube. 


256  APPLIED  BIOLOGY 

Sterilization.  —  It  is  probable  that  both  the  bread  inside  the  tubes 
and  the  cotton  have  spores  of  molds  on  their  surfaces,  because  the 
spores  are  so  abundant  in  ordinary  buildings  that  practically  every- 
thing exposed  to  air  will  have  them.  It  is  therefore  necessary  to  kill 
all  spores  inside  the  plugged  tubes,  that  is,  to  sterilize  or  to  make  the 
tubes  sterile.  This  is  best  done  by  boiling  or  steaming  (100°  C.) 
in  a  sterilizer.  There  are  many  sterilizers  for  laboratory  and  home 
use  on  the  market,  but  one  can  be  easily  made  from  any  tin 
bucket  with  a  cover.  A  sheet  of  perforated  tin  (e.g.,  an  inverted 
pan  two  inches  deep),  or  a  piece  of  wire-netting,  should  be  supported 
about  two  inches  from  the  bottom,  and  on  this  place  the  tubes  to  be 
sterilized.  Put  in  water  two  inches  deep.  For  convenience  in 
keeping  mouths  of  tubes  upward,  they  may  be  tied  into  bunches, 
or  placed  in  small  tin  cans  perforated  with  numerous  holes.  A  layer 
of  cloth  or  cotton  between  the  glass  tubes  and  the  metal  will  prevent 
breakage. 

Keep  the  water  boiling  (100°  C.)  for  a  half -hour.  From  time  to 
time  add  more  water  (hot)  so  that  the  sterilizer  will  not  "  boil  dry." 

Label  some  of  the  tubes  "sterilized  once,  30  minutes."  Set 
aside  for  observation  from  day  to  day.  Do  any  molds  appear  on  the 
bread  in  these  ?  Conclusions  ? 

Sterilize  the  other  tubes  again  two  days  after  the  first  sterilizing, 
and  label  "sterilized  twice,  total  60  minutes."  Set  some  of  these 
aside  and  observe  from  day  to  day.  Sometimes  a  third  sterilizing 
is  necessary  to  kill  all  the  spores  of  molds. 

Inoculation.  —  Take  a  tube  which  two  or  three  days  after  the 
second  or  third  sterilizing  shows  no  sign  of  molds  on  the  bread,  and 
hence  is  probably  sterile,  and  inoculate .  it  as  follows :  Sterilize  an 
inoculating  needle  (or  a  hat-pin,  or  piece  of  wire)  by  passing  it 
quickly  several  times  through  the  flame  of  a  gas-  or  alcohol-lamp. 
Allow  the  needle  to  cool  for  a  moment,  and  then  touch  the  sterilized 
end  carefully  to  a  spot  on  moldy  bread  where  there  is  only  one  kind 
of  mold.  Now,  holding  one  of  the  sterile  tubes  in  a  horizontal 
position,  quickly  pull  out  the  cotton  plug,  insert  the  needle,  wipe  it 
along  one  side  of  the  piece  of  sterile  bread,  and  then  quickly  replace 
the  plug.  Inoculate  other  tubes  from  other  spots  of  molds,  of 
different  kinds  if  available,  but  take  care  to  heat  the  wire  of  the 
inoculating  needle  before  and  after  each  tube  is  inoculated,  otherwise 
you  may  mix  several  kinds  of  spores.  Label  tubes  "black  mold," 
"green  mold,"  etc. 

Do  not  be  surprised  if  with  all  these  precautions  two  kinds  of 
molds  appear  in  some  tubes ;  for  spores  may  have  been  left  alive 


<ii¥.->:'.t!;*).'fl  I  ' 


STUDIES  OF  SPORE-PLANTS  257 

on  the  bread  even  after  sterilizing,  they  may  have  fallen  in  when  the 
tube  was  opened  for  inoculating,  or  more  than  one  kind  of  spores 
may  have  been  on  the  moldy  spot  touched  with  the  needle.  If  only 
one  kind  appears,  a  " pure  culture"  has  been  secured. 

Take  two  tubes  which  have  been  sterilized  twice,  remove  cotton 
plugs,  and  blow  into  them  some  dust  from  top  of  furniture,  door- 
frame, or  window-casing ;  or  sterilize  an  inoculating  needle  and  with 
it  transfer  some  of  the  dust  to  the  sterile  bread.  Record  results 
and  conclusions.  Why  do  foods  mold  quickly  in  a  dusty  pantry  ? 

Leave  the  plugs  out  of  two  sterile  tubes  for       

several  hours,  and  then  replace.     If  molds  de- 
velop, consider  the  possible  source  of  the  spores. 

A  better  way  for  this  experiment  is  to  use  a 

n   *    T  i     i  r»  A  •  j«  i.          j     n  i       FIG.  82.     Section  of  a 

flat  dish,  known  as  a  Petn  dish,  made  for  such         Petri  dish  for  grow_ 

experiments  (Fig.  82).  Place  a  slice  of  moist  ing  molds' and  bac- 
bread  in  such  a  dish,  sterilize  twice,  wait  several  teria.  c,  glass  cover, 
days  to  make  sure  that  the  bread  is  sterile,  The  dots  in  bottom 
then  remove  the  cover  for  several  hours  so  as  sh™  position  of 
.,  »i  i_  j  j_  J.T.  a  J.T.  gelatin,  bread,  or 

to  expose  the  sterile  bread  to  the  air  of  the         other  food 
schoolroom. 

A  small  tumbler  or  wine-glass  may  be  used  instead  of  a  Petri  dish. 
Place  a  piece  of  moist  bread  inside  on  the  bottom  of  the  tumbler, 
cover  with  a  circular  sheet  of  cotton- wadding  which  by  two  inches 
exceeds  the  diameter  of  the  top  of  the  tumbler,  fold  the  edges  of  the 
cotton  down,  and  snap  a  rubber  band  around  it  so  as  to  hold  it  close 
to  the  top  of  the  tumbler.  Sterilize  as  in  case  of  the  Petri  dish. 

A  wide-mouthed  fruit-jar  might  be  used  in  the  same  way,  sealing 
the  jar  with  the  rubber  as  in  canning  fruit.  The  Petri  dish  will  by  con- 
trast show  that  the  rubber  is  not  necessary  to  keep  out  spores.  Also, 
the  Petri  dish  admits  air.  The  secret  is  that  the  spores  respond  to 
gravitation  and  do  not  fall  upward  as  they  must  do  in  order  to  fall 
into  a  Petri  dish.  A  fruit-jar  could  be  used  with  the  cap  and  no 
rubber,  if  the  contents  were  thoroughly  sterilized,  the  edge  of  the 
cap  kept  dry  and  never  turned  upside  down.  But  in  ordinary 
household  canning  of  fruits  the  sterilization  is  not  perfect  (usually 
only  once),  and  the  keeping  out  of  air  by  the  rubber  tends  to  prevent 
the  development  of  molds  which  require  air  in  order  to  grow.  Also, 
the  rubber  prevents  hyphae  from  starting  on  the  outside  and  growing 
beneath  the  cover  into  the  contents  of  the  jar.  This  would  surely 
happen  if  fruit  juice  ran  down  around  the  edge  of  the  cover,  for  the 
juice  would  furnish  excellent  food  for  the  growing  hyphae.  This  is  why 
fruit  in  jars  often  remains  sterile  for  months  and  even  years,  and  then 


258  APPLIED  BIOLOGY 

suddenly  shows  molds.  The  rubbers  have  softened,  or  loosened,  or 
become  moist,  so  that  mold  hyphae  have  grown  into  the  jars.  Hence 
jars  of  fruit  should  be  kept  in  a  cool  and  dry  pjace  unfavorable  for 
growth  of  molds.  Aside  from  this  relation  to  molds,  rubbers  of  fruit- 
jars  also  prevent  evaporation  of  the  fruit  juices. 

Rotting  of  fruit  has  been  mentioned  as  due  to  molds.  Take  a 
perfect  apple  and  inoculate  it  by  sticking  in  several  places  with  a 
needle  which  has  been  plunged  into  a  very  rotten  apple.  Take 
another  perfect  apple,  bruise  the  skin  on  one  side,  and  place  it  in  a 
covered  can  or  box  in  contact  with  a  rotting  apple.  In  both  cases  be 
sure  there  is  moisture  —  the  apples  may  be  kept  on  wet  cloth  or  cotton. 
Take  another  perfect  apple,  gently  rub  the  skin  in  several  places 
with  a  smooth  stick  which  has  been  plunged  into  a  rotting  apple,  so 
as  to  distribute  spores  over  the  uninjured  surface  of  the  apple, 
and  set  this  one  in  a  cool  and  dry  place  for  comparison  with  the  others. 
What  are  the  conclusions  regarding  apples  infecting  each  other? 
Why  should  injured  apples  be  separated  from  perfect  ones  before 
packing  ?  Why  are  apples  bored  by  worms  (larvae  of  codling  moth) 
and  "windfalls"  so  liable  to  decay?  Why  should  fruit-cellars  be 
cool  and  dry?  Since  the  spores  usually  mature  after  rotting  is 
well  advanced,  it  is  evident  that  removal  of  windfall  fruits,  as  by 
pigs  living  in  orchards,  prevents  the  enormous  multiplication  and 
distribution  of  spores  of  various  molds  which  injure  fruit. 

Instead  of  pieces  of  bread  called  for  in  above  experiments, 
a  dilute  sugar  solution  (three  or  four  tablespoonf uls  of  sugar  in  a  pint 
of  water)  may  be  used.  Or  use  some  diluted  juice  from  canned 
fruits.  It  is  interesting  to  prepare  some  tubes  with  the  sugar 
solution  and  try  all  the  experiments  both  with  sugar  and  bread. 
Of  course,  any  other  organic  material  on  which  molds  grow  could 
be  used  in  place  of  bread  or  sugar  solution. 

Write  a  short  essay  on  "Molds  in  Relation  to  Preservation  of 
Food  for  Human  Use."  Read  Section  I  in  Conn's  "Bacteria,  Yeasts, 
and  Molds  in  the  Home." 

246.  Economic  Relations  of  Molds  and  their  Allies.  — 
Many  fungi  popularly  known  as  molds,  mildews,  rusts,  rots, 
blights,  smuts,  scabs  —  all  of  them  more  or  less  closely  related 
to  the  common  molds  which  grow  on  bread  and  other  foods  — 
are  very  important  economically,  because  of  their  injurious 
effect  on  many  cultivated  plants.  The  following  are  common 
examples. 


STUDIES  OF  SPORE-PLANTS  259 

The  common  molds  (black,  green,  brown)  make  preserva- 
tion of  some  foods  for  human  use  difficult,  for  as  the  experi- 
ments have  shown,  brief  exposure  to  the  boiling  temperature 
does  not  kill  the  spores. 

During  a  rainy  season  the  mildews  form  a  white  coating 
on  books,  paper,  clothing,  carpets  and  other  organic  objects 
in  our  homes.  Other  mildews  form  their  mycelia  on  the 
lower  sides  of  many  leaves,  and  some  species  cause  serious 
diseases  of  the  plants.  The  mildews  attacking  grapes  grow 
on  the  leaves  and  fruit  and  cause  enormous  financial  loss 
in  America  and  Europe.  Mildews  often  destroy  lettuce 
and  other  vegetables,  and  severely  injure  gooseberries  and 
other  small  fruits. 

The  rusts,  so-called  because  they  appear  as  streaks  or 
patches  of  yellow  or  black  color  on  leaves  and  other  parts  of 
some  plants,  are  very  destructive.  Rust  of  the  cereal  grains, 
particularly  oats  and  wheat,  causes  damage  in  this  country 
to  the  amount  of  more  than  fifteen  million  dollars  every 
year.  The  rusty  yellow-brown  spots  seen  on  wheat  leaves 
in  early  summer  are  masses  of  spores  from  mycelia  in  the 
tissue  of  the  leaf.  These  spores  are  distributed  by  wind, 
and  thus  infect  other  plants  in  the  same  field.  The  food- 
supply  of  the  rust  is  drawn  from  the  sap  of  the  wheat  plant, 
with  the  result  that  food  for  the  proper  formation  of  grain 
is  not  available.  Some  kinds  of  rusts  require  two  plants 
as  hosts  during  their  life-history.  As  an  example,  one  kind 
of  wheat  rust  has  spores  that  germinate  on  barberry  leaves 
in  the  spring,  and  on  the  barberry  plant  the  rust  produces 
special  spores  which  infect  wheat  plants.  Of  course,  this 
is  not  the  only  kind  of  wheat  rust,  for  in  many  parts  of  this 
country  barberry  bushes  are  rarely  found.  Wheat  may 
suffer  from  rusts  which  require  no  other  plant  for  completing 
their  life-history. 

Smuts.  —  Everybody  who  has  cultivated  Indian  corn 
must  have  noticed  the  "  smutty  "  ears  and  tassels,  on  which 


260  APPLIED  BIOLOGY 

the  smut  plants  appear  first  as  white  masses ;  but  soon  these 
burst  open  and  expose  masses  of  spores.  These  have  the 
blackness  of  lamp-black  or  soot;  hence  the  name  "  smut." 
These  spores  may  infect  other  corn  plants  in  the  same  season. 
Examination  of  a  smutty  ear  of  corn  shows  that  the  grains 
have  been  destroyed  wherever  the  mycelium  of  the  smut 
has  penetrated  them. 

The  blights,  rots,  and  scabs  are  numerous  and  destructive. 
Common  examples  are  potato  blight  of  the  leaves  and  rot 
of  the  tubers  (caused  by  mycelium  growing  from  leaves 
through  stem  to  tubers) ;  the  blight  which  causes  leaves  of 
many  orchard  trees  and  vegetables  to  wither  and  dry  up 
in  part  or  whole ;  peach-leaf  curl ;  the  black  rots  of  grape  and 
tomato ;  the  scabs  of  apple,  grape,  and  potato ;  the  leaf- 
spot  of  pear  and  other  trees ;  and  black  knots  of  plum  and 
cherry  branches. 

The  above-mentioned  examples  are  but  a  small  fraction 
of  the  plants  affected  injuriously  by  fungous  diseases.  Most 
of  the  enemies  of  cultivated  plants  are  either  these  diseases 
or  insects.  One  who  attempts  to  grow  any  special  kind  of 
crop  should  become  familiar  with  the  known  diseases  and 
the  remedies.  For  such  information  consult  the  bulletins  of 
the  United  States  Department  of  Agriculture  and  of  state 
experiment  stations.  Farmers'  Bulletins  91,  219,  243,  250, 
284  (all  free)  describe  some  important  fungous  diseases. 

Remedies.  —  In  general,  the  remedies  against  fungous 
diseases  consist  in  (1)  destroying  all  infected  materials,  such  as 
leaves  and  stems  which  may  carry  spores ;  (2)  in  destroying 
spores  on  seeds  before  planting  (e.g.,  soaking  wheat  and  rye 
in  formalin  solution  to  kill  smut  spores,  and  potato  tubers 
to  kill  potato  rot  or  blight) ;  and  (3)  spraying  the  stems 
and  foliage  with  solutions  containing  lime,  copper  sulphate, 
copper  carbonate,  or  iron  sulphate.  The  spraying  method 
has  within  the  past  two  decades  come  into  extensive  use,  and 
hundreds  of  special  machines  have  been  invented  for  spreading 


STUDIES   OF  SPORE-PLANTS  261 

the  liquids  over  plants  in  fine  mist  so  as  to  touch  all  parts.  A 
mixture  of  lime  and  copper  sulphate,  called  Bordeaux  mixture 
because  discovered  in  about  1885  by  scientists  who  tried 
to  check  the  mildew  which  was  destroying  the  vineyards 
near  Bordeaux  in  France,  has  proved  an  important  remedy  for 
most  fungous  diseases  which  are  on  the  surface  of  plants, 
and  is  extensively  used  by  growers  of  fruits,  potatoes,  many 
vegetables,  and  ornamental  plants. 

Since  insects  may  attack  the  same  plants,  an  insecticide 
(insect-killer)  is  often  mixed  with  the  fungicide.  For  ex- 
ample, Bordeaux  mixture  (for  the  potato  blight  and  rot)  and 
Paris  green  or  other  poisonous  compounds  of  arsenic  (for  kill- 
ing the  potato-beetles)  may  be  mixed.  Likewise,  Bordeaux 
mixture  (for  apple  scabs,  rots,  and  blights  of  either  the  fruit 
or  the  leaves)  and  arsenical  poisons  (for  the  larvae  of  codling 
moth,  which  makes  wormy  apples)  are  commonly  sprayed  as 
a  mixed  liquid. 

For  information  concerning  the  proper  solutions  and  times 
for  spraying  a  particular  crop  in  order  to  get  best  results 
write  to  the  agricultural  departments  either  at  Washington 
or  at  a  state  experiment  station  and  apply  for  the  latest 
pamphlets  on  sprays  for  plant  diseases.  Also  every  spring 
the  best  agricultural  newspapers  review  the  latest  methods 
for  preventing  plant  diseases. 

Water-molds.  —  Some  aquatic  molds  live  on  dead  animals 
and  plants  in  water ;  and  some  of  them  are  very  destructive 
parasites  on  living  fishes.  The  fluffy  white  masses  often 
seen  around  dead  insects  in  water  and  on  fish  in  aquaria  are 
examples.  The  spores  lodge  on  the  skin  of  fishes,  germinate, 
and  some  hyphse  penetrate  the  tissues  of  the  fish  so  that 
nutriment  is  absorbed  from  the  blood  and  lymph.  It  has 
been  found  that  brief  immersion  of  trout  and  other  fishes 
in  sea-water  will  check  the  growth  of  the  fungus. 

Fungi  and  Diseases.  —  Some  of  the  fungi  allied  to  common 
molds  may  produce  diseases  in  man  and  domesticated 


262  APPLIED  BIOLOGY 

animals.  The  danger  from  poisonous  mushrooms  is  well 
known.  Certain  molds  may  grow  in  the  lungs  of  birds, 
other  animals,  and  very  rarely  in  man.  The  smut  of  rye 
and  other  plants  produces  a  poisonous  substance  known  as 
ergot,  which  sometimes  poisons  farm  animals.  In  very 
small  quantities  ergot  is  used  in  medicine.  There  is  always 
danger  in  the  use  of  moldy  foods  by  men  or  farm  animals, 
for  molds  often  form  poisonous  substances  under  conditions 
not  yet  understood  in  science.  There  is  good  reason  for 
thinking  that  a  large  number  of  small  chicks  are  killed  each 
year  by  moldy  food ;  and  several  scientists  believe  that  the 
mysterious  disease  pellagra,  which  has  recently  attracted 
so  much  attention  in  the  southern  states,  is  caused  by  some 
form  of  mold  growing  in  corn  meal,  which  is  extensively 
used  as  food.  Certainly  it  is  best  not  to  use  corn  meal  or 
other  cereal  foods  if  there  is  about  them  the  least  evidence 
of  mold  or  a  musty  odor,  which  indicates  some  kind  of 
molding. 

Other  diseased  conditions  due  to  molds  are  thrush,  a 
disease  of  the  mouth-cavity  of  infants;  ringworm,  often 
seen  on  human  faces  and  scalps;  and  other  skin  diseases 
caused  by  mold-like  plants  whose  mycelia  grow  in  the  human 
tissues.  Most  of  these  are  easily  prevented.  The  spores 
of  all  the  mold-like  plants  which  cause  skin  diseases  may  be 
distributed  by  towels,  sponges,  combs,  and  barbers'  tools. 
Such  articles  should  be  carefully  washed  in  boiling  water 
and  strong  soap  after  being  used  by  diseased  persons.  The 
laws  in  many  states  now  require  such  precautions  in  barber- 
shops. 

Useful  Molds.  —  So  many  of  the  molds  and  their  allies 
are  injurious  from  our  human  point  of  view  that  we  must 
guard  against  the  erroneous  impression  that  all  relatives 
of  the  molds  are  harmful.  The  following  examples  will 
suggest  some  useful  aspects  of  the  life  of  these  lower  fungi. 

The  common  molds  which  cause  decay  oftentimes  destroy 


STUDIES  OF  SPORE-PLANTS  263 

materials  useful  as  human  foods  and  in  other  ways,  but  the 
same  decay  processes  reduce  useless  organic  matters  to  a 
condition  which  makes  them  available  for  plant  food.  For 
example,  the  vast  quantities  of  leaves,  stems,  and  other  plant 
materials  seen  in  late  summer  must  be  decayed  before  their 
elements  can  be  used  over  again  by  plants,  and  in  this 
decay  many  of  the  decomposing  fungi  aid. 

Moreover,  some  fungi  are  useful  because  they  destroy 
insects.  House-flies  are  sometimes  seen  with  delicate 
white  threads  projecting  from  their  bodies.  These  threads 
bear  spore-cases  and  a  mycelium  grows  inside  the  flies. 
The  spores  which  fall  on  other  flies  germinate,  and  so  the 
disease  may  soon  become  epidemic.  Other  similar  fungous 
diseases  attack  other  insects,  and  millions  of  grasshoppers, 
chinch-bugs,  and  other  destructive  species  are  thus  killed. 
An  attempt  has  been  made  to  cultivate  some  of  these  insect 
diseases  artificially  in  order  to  infect  a  few  insects  before 
turning  them  loose  to  spread  the  diseases  to  other  insects. 
The  chief  difficulty  is  in  selecting  weather  favorable  for 
development  of  the  fungi. 

It  should  be  credited  to  the  molds  that  some  of  them 
produce  the  peculiar  flavors  of  cheeses,  such  as  Roquefort 
and  Camembert.  The  manufacturers  take  particular  pains 
to  cause  the  growth  of  almost  a  pure  culture  of  the  proper 
kind  of  mold. 

But  although  some  of  the  molds  and  their  allies  may  be 
useful  from  the  standpoint  of  human  interests,  it  must  be 
admitted  that  their  usefulness  is  far  overbalanced  by  their 
harmfulness,  especially  to  the  numerous  plants  which  are 
important  to  the  human  food-supply. 

Mushrooms 

247.  Molds  have  been  studied  as  examples  of  lower 
and  simple  fungi.  The  mushrooms  are  more  complicated, 


264  APPLIED  BIOLOGY 

and  yet  are  made  up  of  hyphse  and  have  other  character- 
istics showing  relationship  to  molds. 

The  term  mushroom  is  here  used  as  a  general  term  includ- 
ing all  forms  popularly  known  as  mushrooms  and  toad- 
stools, for  there  is  no  scientific  distinction  between  them. 
Formerly  all  edible  fungi  were  called  mushrooms,  and  all 
others  toadstools;  but  many  so-called  toadstools  are  now 
known  to  be  edible.  Practically  and  scientifically,  then,  the 
two  words  are  now  used  quite  synonymously. 

Structure  of  a  Mushroom.  —  (L)  Any  common  mushroom  or 
toadstool  will  serve  for  this  study.  Specimens  may  be  collected 
when  available  and  preserved  in  5  per  cent  formalin-solution.  Attempt 
to  identify  the  points  of  structure  described  below,  and  make  sketches 
in  note-book. 

Examine  a  well-expanded  specimen.  Above  ground  it  consists 
of  a  stalk  and  a  cap.  On  the  under  side  of  the  cap  in  most  species  are 
the  gills,  delicate  plates  radiating  from  the  center.  In  some  species 
there  are  numerous  holes  instead  of  the  gills,  and  in  others  there  are 
projections  in  the  form  of  spines.  If  the  cap  be  cut  from  the  stalk 
and  left  for  a  few  days  with  gills  downward  on  a  sheet  of  white  paper, 
numerous  dark-colored  bodies  (spores)  will  fall  on  the  paper,  leaving 
a  print  of  the  underside  of  the  cap  and  showing  the  arrangement  of 
the  gills.  Closer  examination  of  the  under  side  of  the  cap  will  show 
the  spores  on  the  sides  of  the  gills. 

Below  ground  the  mushroom  is  attached  to  thread-like 
structures  which  are  popularly  called  the  rootlets,  but  they 
have  no  resemblance  to  real  roots  of  higher  plants,  except 
that  they  are  in  the  soil.  These  thread-like  "  rootlets  " 
are  hyphce  and  the  entire  mass  of  them  is  known  as  a  my- 
celium (see  mold,  §244).  These  hyphse  branch  and  grow 
in  the  soil,  and  from  time  to  time  new  mushrooms  or  toad- 
stools grow  upward  from  the  mycelium.  At  first  the  young 
mushroom  is  a  small,  compact,  rounded  body,  concealed  just 
below  the  surface  until  ready  for  expansion,  when  it  absorbs 
water  rapidly  and  may  reach  its  full  growth  in  a  night. 

Microscopic  study  shows  the  stalk  and  cap  to  be  com- 


STUDIES  OF  SPORE-PLANTS  265 

posed  of  closely  packed  and  interwoven  threads  or  hyphae, 
which  have  the  same  structure  as  the  hyphae  below  ground. 
The  part  above  ground  is,  then,  a  sort  of  branch  of  the  sub- 
terranean mycelium. 

Reproduction.  —  A  new  mycelium  is  produced  by  germina- 
tion of  a  spore  which  falls  in  a  favorable  place.  The  stalk 
and  cap  raise  the  spore-producing  hyphae  above  ground  so 
that  the  spores  may  be  more  widely  disseminated. 

"  Fairy-rings."  —  A  mycelium  once  started  in  the  ground 
may  continue  to  grow  and  produce  new  mushrooms  for 
some  time.  Frequently  mushrooms  are  seen  in  pastures 
arranged  in  circles.  These  were  once  popularly  supposed 
to  have  been  caused  during  the  night  by  fairies  in  their 
dances,  and  were  called  fairy-rings,  or  fairy-circles.  This  in- 
teresting explanation  was  especially  applicable  to  rings  formed 
by  certain  minute  relatives  of  the  mushroom  which  give  the 
soil  the  appearance  of  having  been  sprinkled  with  ashes. 
But  science  has  in  many  ways  given  us  rational  explanations 
of  many  puzzling  things  in  nature,  and  we  no  longer  need 
to  assume  the  existence  of  fairies  and  other  mythical  beings. 
Scientific  study  of  the  "  fairy-circles  "  long  ago  showed  that 
they  are  caused  by  the  regular  growth  of  a  mycelium  from  a 
center,  and  as  the  older  and  central  portions  die,  the  mush- 
rooms will  appear  in  continually  widening  circles. 

Cultivated  mushrooms  are  grown  from  "  spawn  "  which 
comes  from  seed  dealers  in  the  form  of  dry  bricks  composed 
of  partially  decayed  vegetable  matter.  The  spawn  is  really 
nothing  but  mycelium,  either  taken  from  old  mushroom 
beds,  or  started  by  scattering  spores  over  materials  out  of 
which  the  bricks  are  to  be  made  as  soon  as  the  spores  have 
germinated  and  formed  an  extensive  mycelium.  It  is  simply 
necessary  to  break  such  bricks  into  pieces  and  plant  them  in 
a  bed  of  suitable  soil  with  abundance  of  decaying  organic 
matter.  Concerning  the  cultivation  of  mushrooms  see 
Farmers'  Bulletin  No.  204  (free). 


266  APPLIED  BIOLOGY 

Ants  raising  Mushrooms. — One  of  the  interesting  members 
of  the  mushroom  group  is  a  species  which  is  regularly  culti- 
vated by  the  leaf -cutting  ants  of  the  tropics.  Pieces  of  leaves 
are  chewed  by  the  ants  and  then  packed  away  to  decay  and 
form  a  proper  substratum  for  the  growth  of  a  mycelium, 
which  is  eaten  by  the  ants. 

Physiology  of  Mushroom. — As  described  in  §  98,  plants  with- 
out chlorophyll  are  like  animals  in  that  they  must  get  their 
food  from  other  plants,  either  as  parasites  on  living  plants 
(e.g.,  dodder),  or  as  saprophytes  on  decaying  plant  matter. 
The  common  mushrooms  are  usually  saprophytes,  but  some 
of  them  grow  as  parasites  on  living  trees  (e.g.,  shelf-fungi). 

In  their  breathing  mushrooms  are  like  animals  in  that 
they  take  in  oxygen  and  excrete  carbon  dioxide. 

248.  Lichens.  —  The  familiar  plants  known  as  lichens, 
which  in  the  form  of  grayish  scales  or  moss-like  masses 
grow  on  rocks  and  bark  of  trees,  consist  of  two  kinds  of  plants 
living  together  in  intimate  connection.  One  of  the  plants 
is  a  member  of  the  Algae  (§  236),  and  able  to  make  carbohy- 
drates from  carbon  dioxide  and  water;  while  the  other  is 
a  fungus,  which  obtains  its  carbohydrate  food  from  its 
associate.  The  main  mass  of  a  lichen  is  a  dense  mycelium 
similar  in  microscopic  structure  to  some  mushrooms,  and 
in  this  are  the  cells  or  filaments  of  the  associated  green  plant. 

Such  a  living  together  is  an  example  of  symbiosis.  A 
similar  association  between  a  plant  and  an  animal  is  described 
in  §  285. 

It  is  possible  to  collect  the  spores  and  grow  separately 
the  two  kinds  of  plants  included  in  a  lichen. 

Reindeer-moss  (a  valuable  food  for  reindeer)  and  Ice- 
land moss  (used  in  cooking)  are  lichens.  Litmus  (used  for 
testing  acids  and  alkalies)  comes  from  a  lichen.  On  the 
mountains  lichens  assist  in  weathering  rocks,  and  thus  form- 
ing soil.  An  edible  form  found  on  sandy  deserts  is  called 
manna-lichen. 


STUDIES   OF  SPOBE-PLANTS  267 

249.  Economic  Relations  of  Mushrooms.  —  Food  Value. 
The  words  "  economic  relations  "  will  probably  lead  most  read- 
ers to  think  instantly  of  edible  mushrooms.  There  are  many 
thousand  species  of  the  group  to  which  the  common  mush- 
rooms belong,  and  many  of  them  are  edible.  The  fact  is 
that  thay  have  little  value  as  human  food,  but  they  are  deli- 
cious as  relishes.  The  wild  forms  are  not  commonly  used 
because  of  the  difficulty  of  distinguishing  with  certainty 
between  edible  and  poisonous  species.  There  is  no  safe 
general  rule;  and  botanists  who  have  specially  studied 
mushrooms  advise  that  no  species  should  be  eaten  unless 
identified  by  one  who  is  competent.  Especially  should 
people  avoid  mushrooms  with  a  ring  or  cup  at  the  base,  for 
this  is  one  mark  of  the  deadly  Amanita,  one  of  the  most 
poisonous  plants  known  to  science.  Such  books  as  Atkinson's 
"  Mushrooms  "  ;  Marshall's  "  Mushroom  Book  "  ;  "  Mush- 
room Poisoning" — a  circular  of  the  United  States  Depart- 
ment of  Agriculture;  and  Farlow's  "  Edible  and  Poisonous 
Mushrooms  "  —  also  issued  by  the  United  States  Depart- 
ment of  Agriculture,  give  pictures  and  descriptions  of  many 
species  pronounced  edible.  But  even  with  these  books  one 
should  be  exceedingly  cautious  before  deciding  to  eat  a  mush- 
room which  resembles  those  said  to  be  poisonous. 

Effect  on  Timber.  —  More  important  economically  than 
their  food  value  is  the  destructive  action  of  some  mushrooms 
on  trees  valuable  for  lumber.  In  sawing  trees  into  lumber 
the  heart-wood  is  often  found  filled  with  white  threads 
(mycelium)  which  have  so  softened  the  wood  as  to  make  it 
worthless.  Hollow  trees  are  usually  due  to  such  decay.  The 
mycelium  seen  in  the  wood  is  connected  with  a  shelf-like 
structure  (often  called  shelf-mushroom  or  shelf-fungus) 
on  the  bark  of  trees.  In  walking  through  woodland  one  can 
easily  find  pieces  of  dead  branches  with  small  toadstools 
attached,  and  if  these  be  split  open,  the  white  thread-like 
mycelium  may  be  seen.  These  threads  (hyphse)  enter  trees 


268  APPLIED  BIOLOGY 

through  some  injured  tissue,  as  a  broken  branch,  gnawing  by 
animals,  or  careless  pruning  which  leaves  large  surfaces 
exposed.  The  reason  for  painting  all  injured  surfaces  of 
trees  with  tar,  cement,  or  paint  is  that  these  substances  will 
keep  out  mycelia.  Spores  are  abundant  in  the  air  and  may 
at  any  time  fall  upon  an  injured  surface,  germinate,  and  form 
a  mycelium  which  penetrates  between  the  cells  of  the  wood. 
It  is  an  easy  matter  to  keep  mycelia  from  entering  a  tree 
at  an  injured  place,  but  quite  impossible  to  check  them  when 
once  they  have  penetrated  deeply  into  the  plant. 

The  "  dry  rot "  which  often  attacks  the  foundation  tim- 
bers of  houses  and  makes  them  unable  to  stand  the  strain  of 
supporting  buildings  is  due  to  the  mycelium  of  a  near  rela- 
tive of  common  mushrooms.  The  remedy  for  "  dry  rot " 
is  ventilation  of  spaces  beneath  buildings  and  coating  tim- 
bers with  tar,  crude  oil,  or  creosote. 

Some  mushrooms  have  their  mycelia  penetrating  the  roots 
of  trees,  causing  decay  and  weakening  so  that  heavy  winds 
uproot  them. 

Puff-balls,  named  because  they  puff  out  a  mass  of  spores 
when  ripe,  are  near  relatives  of  the  common  mushrooms. 
Their  mycelia  grow  in  rotten  logs  and  humus  (decaying 
vegetable  matter) .  They  contain  numerous  small  cavities  in 
which  spores  are  formed.  Some  of  the  puff-balls  are  edible 
in  the  young  state.  One  kind  of  puff-balls,  called  earth- 
star,  has  its  outer  skin  split  to  form  a  number  of  star-like 
projections. 

References  for  pupils.  —  In  addition  to  those  given  in  the  text 
above,  Atkinson's  "  Elementary  Botany,"  pp.  326-339  ;  Marshall's 
"  Mushroom  Book "  (excellent  illustrations)  ;  Coulter's  "  Plant 
Structures,"  pp.  68-74. 

Yeast  Plants 

250.  The  Cause  of  Fermentation.  —  It  is  a  well-known 
fact  that  liquids  containing  sugar,  such  as  juices  of  grapes, 


STUDIES  OF  SPORE-PLANTS 


269 


apples,  and  other  fruits,  commonly  undergo  a  change  called 
fermentation.  The  result  of  this  process  is  the  production 
of  an  invisible  gas  (carbon  dioxide),  which  causes  efferves- 
cence of  the  fermenting  liquid,  and  alcohol.  No  other  method 
of  producing  alcohol  is  known  to  occur  in  nature;  and  so 
for  thousands  of  years  fermentation  has  been  used  in  making 
wines  and  other  alcoholic  beverages  from  fruit  juices.  It 
was  not  until  the  nineteenth  century  that  the  microscopic 
yeast  plants  were  recognized  as  the  cause  of  fermentation; 
and  the  experiments  performed  by  the  great  French  naturalist, 
Louis  Pasteur,  between  1855  and  1865  will  always  be  famous 
as  having  placed  our  knowledge  of  fermentation  processes  on 

a  thoroughly  scientific  foundation. 
i 

251.  Study  of  Yeast  Plants.  —  (L)  Scrape  some  small  pieces  from 
a  cake  of  compressed  yeast,  mix  with  a  drop  of  water  on  an  object- 
slide,  and  gently  lower  a  coverrglass 
into  position.  Also,  if  brewer's  or 
baker's  yeast  in  liquid  form  is  avail- 
able, mount  a  drop  on  another  slide. 
Prepare  a  third  slide  by  mounting  a 
drop  of  molasses-solution  )  water  10 
parts,  common  dark-colored  molasses 
1  part)  in  which  some  scrapings  of  com- 
pressed yeast  or  some  drops  of  brewer's 
yeast  were  placed  on  the  previous  day 
and  the  liquid  kept  in  a  warm  place 
(70°  to  90°  F).  It  is  best  to  draw  up 
with  a  rubber-bulbed  pipette,  or  with  a 
dipping  tube,  some  of  the  white  sedi- 
ment in  the  bottom  of  the  bottle  con- 
taining the  molasses-solution. 

Examine  the  slides  with  a  high  power 
of  microscope.  Small  oval  translucent 
bodies,  some  isolated,  some  united  into 
groups  or  chains,  will  be  seen.  Each 
oval  body  is  a  yeast  plant ;  and  it  is  a 

single  cell.  The  chains  are  due  to  reproduction  by  formation  of 
buds.  By  comparing  the  sizes  of  the  cells  in  a  chain,  it  is  possible  to 
determine  the  order  in  which  the  buds  were  formed  (Fig.  83).  Soon 


FIG.  83.  Yeast  cells,  two  cells 
beginning  to  bud,  and  three 
chains  of  cells,  formed  by 
repeated  budding.  Clear 
centers  are  cavities.  Nuclei 
not  shown.  (From  Sedgwick 
and  Wilson.) 


270  APPLIED  BIOLOGY 

the  cells  of  a  chain  become  isolated,  and  each  may  form  a  chain  of 
new  cells  when  food  for  growth  is  abundant.  An  abundance  of  long 
chains  indicates  that  the  yeast  has  been  growing  rapidly  ;  compare 
that  taken  directly  from  the  compressed  yeast  with  that  which  has 
been  in  molasses-solution. 

The  yeast  cells  are  variable  in  size  and  shape.  The  con- 
tents consist  of  protoplasm,  cavities  containing  a  clear 
fluid,  small  droplets  of  oil,  and  a  nucleus  which  is  invisible 
in  ordinary  living  yeast  cells.  The  appearance  is  different  in 
the  "  resting  "  cells,  as  in  the  yeast-cake,  from  that  of  the 
actively  growing  cells.  This  is  visible  evidence  that  changes 
occur  in  protoplasm  when  it  is  especially  active.  Of  course, 
slight  changes  are  taking  place  constantly  as  long  as  the 
yeast  cells  are  living,  but  during  the  great  activities  attendant 
upon  growth  the  changes  become  distinctly  visible.  Many 
such  observations  on  living  transparent  cells  of  lower  organ- 
isms have  helped  biologists  gain  very  important  knowledge 
of  cell-life. 

Experiments  with  Yeast.  —  (D  or  L)  In  the  following  experiments 
the  amount  of  growth  may  be  roughly  estimated  by  the  turbidity  or 
cloudiness  in  the  liquid.  It  may  be  determined  accurately  by 
microscopic  examination,  especially  by  the  number  of  buds  to 
which  each  cell  has  given  rise. 

Experiment  1.  Effect  of  food-supply  upon  growth  and  fermenta- 
tion. Take  seven  test-tubes,  or  small  bottles,  and  fill  one-half  full  as 
follows:  (1)  distilled  water  or  rain-water ;  (2)  10  per  cent  solution  of 
sugar  in  water  (water  500  cc.  or  nearly  1  pint ;  granulated  sugar  50 
grams) ;  (3)  10  per  cent  sugar,  and  add  to  the  tube  a  mass  of  commer- 
cial beef-extract  as  large  as  a  pea ;  (4)  molasses-solution  (any  dark 
colored  molasses,  such  as  New  Orleans,  1  part  to  water  10  parts) ; 
(5)  wheat  flour  in  water  to  make  very  thin  paste;  (6)  Pasteur's 
solution ;  *  (7)  Pasteur's  solution  without  sugar. 


*  Pasteur's  solution  is  water  containing  sugar  (C,  H,  O),  ammonium 
tartrate  (N),  potassium  phosphate  (K,  P),  calcium  phosphate  (Ca,  P),  and 
magnesium  sulphate  (Mg,  S).  The  symbols  in  parentheses  show  the  ele- 
ments (total  nine)  which  the  yeast  plant  gets  from  each  compound  used  in 
making  the  solution,  and  the  amount  of  these  used  was  originally  determined 
by  chemical  analysis  of  yeast. 


STUDIES  OF  SPORE-PLANTS  271 

Tubes  6  and  7  may  be  omitted  if  materials  are  not  at  hand. 
Carefully  label  each  tube  with  the  numbers  (on  paper  labels,  or 
with  wax-pencil).  Put  one  drop  of  washed  yeast  *  into  each, 
shake  the  tubes  thoroughly,  tightly  plug  the  mouth  of  each  with  a  wad 
of  clean  cotton-batting  or  absorbent  cotton,  and  set  them  in  a  warm 
place  near  a  stove  or  radiator. 

Examine  the  tubes  1  to  7  after  several  hours  and  again  on  the  next 
day  and  judge  from  the  turbidity,  and  if  possible  also  from  micro- 
scopic examination,  (1)  in  which  fluids  the  yeast  grows  best, 
and  (2)  in  which  fermentation  takes  place.  In  which  are  the  most 
bubbles  of  gas  formed  ?  Does  the  formation  of  gas  bear  any  relation 
to  the  growth  ?  (Compare  2  and  4.)  Keep  careful  notes,  and  at  the 
close  of  the  experiments  write  an  account  of  them,  giving  your 
observations  and  conclusions  regarding  the  substances  necessary 
for  growth  of  yeast  and  for  fermentation. 

Experiment  2.  (D)  Effect  of  heat  on  yeast.  Three  test-tubes 
half  full  with  molasses-solution.  Put  equal  amounts  of  yeast  into 
each,  and  plug  tubes  with  cotton.  Label  1,  2,  3.  Heat  No.  1  to 
the  boiling  temperature  by  holding  tube  in  flame  of  gas-burner, 
and  leave  Nos.  2  and  3  unheated.  Keep  tubes  No.  1  and  No.  2  to- 
gether in  a  warm  place  (between  25  and  35°  C.),  and  No.  3  on  ice  in 
an  ice-box.  After  a  day  or  two  examine  first  as  to  turbidity  and 
indications  of  fermentation  and  then  open  tubes  and  examine  mi- 
croscopically. Write  conclusions  as  to  effect  of  high  and  low 
temperature  on  yeast. 

Experiment  3.  (D)  Gas  evolved  by  yeast  plant.  (1)  Take  one 
large  test-tube,  about  £  by  5  inches,  and  pour  in  lime-  or  barium- 
water  about  £  inch  deep.  Take  a  smaller  tube,  about  I  by  3  inches, 
and  tie  a  thread  around  its  neck.  Provide  a  cork  for  the  larger  tube. 
Pour  vinegar  into  smaller  tube  (\  full),  get  it  ready  to  lower  into 
the  larger  tube.  Then  drop  into  the  vinegar  a  small  lump  of  baking 
soda,  and  by  the  thread  quickly  lower  the  small  tube,  and  stopper 
the  mouth  of  the  larger  one.  Results  ?  Compare  change  of  lime- 
water  with  that  produced  by  breathing  through  a  straw  or  glass  tube 
into  a  bottle  containing  lime-water. 

*  The  washed  yeast  may  be  obtained  by  taking  with  a  long  pipette  some 
white  sediment  from  bottom  of  bottle  with  molasses-solution  in  which  yeast 
has  been  growing  for  several  days.  Wash  this  sediment  by  placing  on  filter- 
paper  (preferably  in  a  funnel)  and  pouring  water  through  it  to  wash  out  the 
soluble  food  (sugar,  etc.).  Then  place  the  paper  in  a  small  bottle  of  water  and 
shake  to  dislodge  the  yeast  cells.  They  settle  to  the  bottom  and  may  be 
easily  transferred  by  a  pipette  to  tubes  of  solutions  mentioned  above. 


272 


APPLIED  BIOLOGY 


(2)  Now,  wash  out  the  tubes  with  clean  water.  Again  put  lime- 
water  in  the  bottom  of  the  larger  tube.  Into  the  smaller  tube 
put  some  molasses-solution  (about  half  full),  and  put  in  some  yeast. 
Lower  the  small  tube  into  the  larger  one,  and  insert  the  stopper. 
Set  in  a  warm  place,  and  after  the  fermentation  has  continued  for 
an  hour  or  more  look  for  changes  in  the  lime-water.  (If  the  teacher 
has  ready  a  pan  of  water,  heated  as  much 
as  the  hand  will  bear  with  comfort,  and 
the  tubes  are  prepared  from  growing  yeast 
at  the  beginning  of  a  two-hour  session 
and  placed  in  the  warm  water,  it  will  be 
possible  to  get  results  before  the  close  of 
the  session ;  but  examine  the  tubes  again 
after  they  stand  a  day  or  more  in  a  warm 
place . )  What  does  this  experiment  prove  ? 
Compare  with  similar  experiments  pre- 
viously performed  (§26). 

Experiment  4.  (D)  Conducting  the  gas 
into  lime-water.  Prepare  apparatus  shown 
in  Fig.  84.  In  tube  a  put  lime-water,  in 
tube  b  put  some  molasses-solution  with 
yeast.  Set  in  a  warm  place,  and  when 
active  fermentation  begins  notice  bubbles 
of  gas  rising  through  the  lime-water  and  its  effect.  Compare  with 
Experiment  3. 

Experiment  5.  (D)  Collecting  gas  from  fermentation.  (1)  Arrange 
apparatus  shown  in  Fig.  85.  In  the  flask  y  put  molasses-solution 
with  yeast;  fill  the 
bottle  with  water  be- 
fore inverting  and 
suspending  in  posi- 
tion shown  in  the 
figure.  Set  in  warm 
place.  When  gas  is 
evolved  from  the 
flask  it  will  be  col- 

FIG.  85.     Apparatus  for  collecting  gas  evolved  by 
yeast,     y,  flask  or  bottle  with  yeast  in  molasses- 


FIG.  84.  Two  glass  bottles 
connected  by  a  bent  glass 
tube  (0,  inserted  through 
the  cork  (c).  Tube  a  con- 
tains yeast  in  molasses-so- 
lution (y) ,  and  tube  b  con- 
tains lime-water  (w). 


solution;  w,  water  partly  displaced  by  gas  (c). 


lected  in  the  bottle, 

the  bubbles  rising 

and    displacing    the 

water.     When  the  bottle  is  full  of  the  gas,  that  is,  when  all  the  water 

has  been  displaced,  carefully  insert  a  stopper  before  raising  the  mouth 

of  bottle  above  the  surface  of  the  water.     (2)  Holding  the  bottle  in 


STUDIES   OF  SPORE-PLANTS  273 

upright  position,  quickly  remove  the  stopper  and  plunge  a  glowing 
taper  into  the  gas.  Result  ?  (3)  Instead  of  testing  with  flame,  the 
invisible  gas  may  be  poured  into  tube  having  lime-water. 

Experiment  6.  (D)  Effect  of  the  gas  on  a  flame.  Take  a  pint 
bottle  or  fruit-jar,  pour  in  some  molasses-solution,  add  yeast,  cork 
tightly,  and  allow  fermentation  to  go  on  for  several  days.  Then 
remove  cork  and  quickly  insert  a  glowing  taper.  Result  ? 

Experiment  7.  (D)  Alcohol  produced  by  fermentation  of  sugar. 
(1)  Put  a  few  drops  of  strong  alcohol  in  a  test-tube  half  full  of  water, 
add  a  crystal  of  iodine,  and  heat  over  gas-  or  alcohol-lamp.  Then 
add  strong  solution  of  caustic  potash,  or  baking  soda,  until  the  iodine 
color  fades.  Notice  the  crystals  and  odor  of  iodoform,  especially 
after  cooling.  This  is  a  test  for  alcohol.  (2)  Take  some  molasses- 
solution  in  which  yeast  has  been  growing  for  several  days,  add 
iodine,  and  test  as  above.  From  a  large  quantity  of  fermented 
molasses  some  alcohol  might  be  collected  by  distilling. 

Experiments.  (D)  Effect  of  dense  sugar  solution.  (1)  Place  some 
yeast  in  undiluted  molasses,  keep  in  a  warm  place,  and  compare  with 
results  previously  obtained  with  molasses-solution.  Write  your 
conclusions  concerning  the  fact  that  fruit  "preserves"  do  not  fer- 
ment. (2)  Take  some  "preserved"  fruit  long  kept  free  from  fer- 
mentation, place  in  test-tube  or  bottle  with  distilled  or  boiled  water, 
add  some  yeast,  keep  in  warm  place.  Results?  Conclusions? 

Experiment  9.  Bubbles  formed  by  gas  from  fermentation.  (D) 
Place  some  yeast  in  molasses-solution  in  a  test-tube,  and  pour  on 
the  surface  of  the  molasses  a  small  quantity  of  flour  paste.  The 
paste  will  form  bubbles  as  the  gas  is  evolved. 

Experiment  10.  Yeast  in  bread-making.  (Optional.)  These 
experiments  may  be  prepared  by  some  pupils  at  home  and  demon- 
strated to  the  class  with  full  explanations.  Mix  some  actively 
growing  yeast  with  flour  and  water  to  make  dough.  Bake  some 
lumps  of  the  dough  at  once  and  others  after  leaving  in  a  warm  place 
until  "rising"  occurs.  At  the  same  time  mix  some  baking  powder 
with  flour  and  water,  and  bake  some  of  the  lumps  of  the  dough. 
Cut  across  the  lumps,  and  compare  the  cut  surfaces.  What  has 
been  the  effect  of  the  yeast?  Of  the  baking  powder?  Compare 
with  the  previous  experiment  with  bubbles. 

Experiment  11.  Wild  yeasts.  (D,  optional.)  (1)  Crush  some 
fruit  (apples  or  grapes),  put  the  juice  into  a  test-tube,  plug  with 
cotton,  and  leave  in  a  warm  place.  (2)  Put  some  molasses-solution 
into  another  tube,  and  leave  exposed  to  the  air  in  a  warm  place. 
Does  fermentation  occur?  Examine  a  drop  of  the  fluid  with  the 


274  APPLIED  BIOLOGY 

microscope.  Are  yeast  cells  present?  Where  could  they  have 
come  from  in  1  and  in  2  ?  Why  does  bottled  cider  or  grape-juice 
ferment  soon  after  a  bottle  is  opened?  Why  do  we  suspect  the 
presence  of  some  antiseptic  when  grape-juice  does  not  ferment  in  an 
open  bottle  ? 

Experiment  12.  "Salt-raising  bread."  (Optional.)  Add  enough 
common  salt  to  make  some  milk  taste  slightly  salty,  and  set  in  a 
warm  place  until  foam  appears  on  the  surface,  i.e.,  fermentation 
begins.  Yeast  cells  come  from  the  air.  The  salt  prevents  the  growth 
of  certain  other  microscopic  organisms  (bacteria)  which  would  cause 
souring  of  the  milk.  Such  fermenting  milk  mixed  with  dough  causes 
it  to  "raise,"  but  less  than  when  yeast  is  used.  Hence,  bread  made 
in  this  way  is  heavier  than  yeast  bread. 

252.  Physiology  of  Yeast  Plant. —The  foregoing  ex- 
periments have  given  information  on  which  we  may  base 
some  general  statements  concerning  the  physiology  of  the 
life-activities  of  the  yeast  plant.  The  plant  is  a  one-celled 
organism,  without  chlorophyll,  and  for  its  food  dependent 
upon  materials  built  up  by  other  plants.  It  is  therefore  a 
saprophyte  like  the  molds,  mushrooms,  Indian  pipe,  and  many 
other  plants  without  chlorophyll.  The  studies  which  Pasteur 
made  with  the  solution  which  bears  his  name  showed  that 
yeast  plants  must  have  nine  elements  in  their  food ;  namely, 
carbon,  hydrogen,  oxygen,  nitrogen,  calcium,  sulphur, 
phosphorus,  potassium,  and  magnesium.  These  are  neces- 
sary for  the  formation  of  new  yeast  protoplasm,  that  is, 
for  growth ;  and  if  fermentation  is  also  to  take  place,  sugar 
(or  starch  convertible  into  sugar)  must  also  be  present. 
Fermentation  results  in  changing  sugar  into  alcohol  and 
carbon  dioxide.  Within  recent  years  it  has  been  demon- 
strated that  this  is  due  to  an  enzyme  secreted  by  yeast  plants. 
Under  great  pressure  this  enzyme  has  been  extracted  and  used 
to  cause  fermentation  without  the  presence  of  living  yeast  cells. 
This  is  in  principle  similar  to  the  diastase  extracted  from 
plant  cells  and  used  in  digesting  starch  to  sugar  in  test- 
tube  experiments,  or  to  pepsin  from  animal  stomachs. 


STUDIES   OF  SPORE-PLANTS  275 

The  usual  method  of  reproduction  is  by  budding,  but 
under  certain  conditions  a  yeast  cell  may  form  spores  within 
itself,  and  later  each  of  these  spores  may  grow  into  an  ordinary 
yeast  cell  capable  of  budding. 

As  to  relation  to  temperature,  the  yeast  plants  grow  best 
at  about  20  to  30°  C.  They  are  quickly  killed  at  the  boil- 
ing temperature,  and  hence  it  is  quite  easy  to  bottle  sweet 
(unfermented)  cider  or  grape  juice.  There  is  apparently 
little  growth  at  the  temperature  of  ice-boxes,  and  hence 
fermentable  foods  may  be  preserved  at  low  temperatures; 
but  only  temporarily,  for  other  organisms  may  develop, 
except  at  the  freezing  temperature  (which  is  colder  than  in 
common  ice-boxes). 

Yeast  plants  can  withstand  partial  drying.  This  explains 
their  living  in  the  cakes  of  dry  yeast ;  but  after  some  months 
all  the  cells  are  dead  in  such  cakes.  It  also  explains  how  the 
wild  yeasts  can  live  in  the  dust  on  fruits,  or  on  the  surface 
of  the  soil  in  orchards  and  vineyards,  ready  to  begin  growth 
as  soon  as  the  bruised  or  crushed  fruit  allows  the  yeast  plants 
access  to  the  juices.  In  the  dry  condition,  the  yeast  cells 
float  with  dust  in  the  air,  and  this  explains  why  sterile  fruit- 
juices  (e.g.,  boiled  cider)  or  sugary  solutions  will  soon  ferment 
after  exposure  to  ordinary  air. 

253.  Economic  Relations  of  Yeasts.  —  It  is  only  neces- 
sary to  allude  briefly  to  the  vast  industries  of  manufacturing 
bread  and  alcoholic  beverages  which  are  dependent  upon  pro- 
duction of  alcohol  and  carbon  dioxide  by  yeast  plants.  So 
great  is  the  importance  of  pure  yeasts  in  these  two  industries 
that  enormous  factories,  costing  immense  sums  of  money,  are 
devoted  to  raising  yeasts. 

Apart  from  alcohol  in  beverages  and  in  medicine,  it  has 
long  been  invaluable  in  varnishes  and  numerous  other  com- 
pounds used  in  mechanic  arts.  Recently  it  has  been  coming 
into  prominence  as  a  cheap  source  of  energy  for  engines, 
burners,  lamps,  etc.  It  is  possible  to  make  alcohol  cheaply 


276  APPLIED  BIOLOGY 

from  much  plant  material  which,  on  the  ordinary  farm,  goes 
to  waste  (unmarketable  fruits  and  grains) ;  and  it  is  probable 
that  before  many  years  pass  the  making  of  alcohol  for  mechan- 
ical purposes  will  be  a  vast  industry  in  agricultural  regions. 

Vinegar,  which  is  used  so  extensively  as  a  condiment  and 
for  preserving  certain  foods  for  human  use,  is  best  made 
from  alcohol  derived  from  fermented  fruit  juice.  For  ex- 
ample, yeast  plants  ferment  the  sugar  of  apple  cider  and  form 
alcohol ;  and  the  vinegar  bacteria,  of  which  there  are  millions 
in  "  mother  of  vinegar,"  ferment  the  alcohol  into  acetic 
acid,  which  gives  vinegar  its  acid  qualities. 

On  the  harmful  side,  we  must  mention  the  undesired  fer- 
mentation of  fruits  and  sugary  mixtures  prepared  for  human 
food.  However,  we  have  seen  that  heating  to  near  boiling 
will  check  such  action  of  yeast.  Sometimes  yeasts  cause 
undesired  fermentation  in  milk  and  cheese  in  dairies. 

Certain  kinds  of  yeasts  (not  the  varieties  used  commonly) 
have  caused  skin  diseases,  but  rarely. 

We  must  conclude,  then,  that  on  the  whole,  yeast  plants 
are  decidedly  useful  as  producers  of  alcohol  and  carbon 
dioxide.  Most  of  the  bread-making  since  human  civilization 
reached  a  stage  where  attention  began  to  be  paid  to  preparing 
the  best  possible  foods,  has  depended  upon  yeast  plants. 
Quite  apart  from  the  question  of  the  harmful  use  of  alcoholic 
liquors,  to  be  discussed  in  the  last  part  of  this  book,  there  can 
be  no  question  concerning  the  value  of  alcohol  for  many 
other  purposes.  The  usefulness  of  yeasts  forms  a  decided 
contrast  to  the  harmfulness  of  the  molds  (§  246). 

Reading  for  pupils:  Section  II  in  Conn's  "Bacteria,  Yeasts,  and 
Molds  in  the  Home." 

III.  BACTERIA 

254.    Bacteria,     Germs,     Micro-organisms,     Microbes.  — 

The  word  bacteria  (singular,  bacterium)  is  the  biological  name 
for  a  group  of  one-celled  plants  without  chlorophyll,  which 


STUDIES  OF  SPORE-PLANTS  277 

are  lower  than  any  we  have  yet  studied.  Since  they  have 
no  chlorophyll,  some  authors  class  the  bacteria  with  fungi; 
but  they  are  probably  not  closely  related  to  yeasts,  molds 
and  other  fungi.  Popularly,  the  four  words  which  stand  in 
the  above  heading  are  used  as  if  synonymous ;  but  the  fact 
is  that  the  last  three  may  properly  be  applied  to  some  micro- 
scopic organisms  which  are  not  bacteria,  and  in  certain  cases 
are  one-celled  animals.  In  other  words,  the  last  three  are 
general  words  meaning  microscopic  organisms ;  while  bac- 
teria are  a  special  kind  of  such  small  organisms,  belonging  to 
the  plant  kingdom.  Sometimes  it  is  very  convenient  to 
have  the  general  terms.  For  example,  the  statement  that 
micro-organisms  affect  our  everyday  life  in  many  important 
ways  includes  other  organisms  as  well  as  bacteria.  Some 
of  these  other  organisms  will  be  referred  to  briefly  at  the 
close  of  this  section,  because  the  practical  relations  to  our 
daily  lives  of  many  of  them  are  similar  to  those  of  the  bacteria. 
From  our  studies  of  yeasts  and  molds  it  is  evident  that 
foods  prepared  for  human  use  are  likely  to  ferment  (that  is, 
favor  growth  of  yeast),  if  in  a  liquid  form  "containing  a  cer- 
tain amount  of  sugar  (e.g.,  fruit  juice,  or  dilute  sugary  syrup). 
If  the  foods  are  solid  and  contain  a  large  amount  of  starch  or 
sugar  (e.g.,  bread  or  preserves),  molds  are  likely  to  develop 
first.  If  the  foods  contain  considerable  protein  (e.g.,  soups, 
such  vegetables  as  peas  and  beans  and  meats),  it  is  a  well- 
known  fact  that  decay  characterized  by  disagreeable  odors 
begins  quickly.  This  change  is  caused  by  bacteria,  which 
may  grow  so  rapidly  as  to  prevent  development  of  the  yeasts 
or  molds.  In  fact,  there  is  a  tendency  among  the  bacteria, 
yeasts,  and  mold?  for  the  kind  which  gets  the  first  start  to 
crowd  out  the  others.  Thus  bacteria  do  not  usually  get  a 
chance  to  develop  in  fruit  juice  until  after  yeasts  have  used 
up  the  sugar  ;  bacteria  get  the  first  start  in  protein  foods; 
and  molds  predominate  on  starchy  foods.  An  apple  may 
first  start  to  rot  because  filaments  or  hyphae  of  molds  pene- 


278  APPLIED  BIOLOGY 

trate  it ;  but  yeasts  and  bacteria  may  develop  later.  This 
opposition  of  different  kinds  of  organisms  is  often  called 
antagonism.  It  is  important,  for  harmless  micro-organisms 
often  prevent  the  multiplication  of  other  kinds. 

255.  Study  of  Bacteria.  —  (L  or  D)  Clean  by  washing  with  soap 
or  soap-powder  and  rinsing  some  small  bottles  or  test-tubes  and  plug 
with  cotton  as  described  in  §  245.  Place  the  plugged  tubes  in  a 
sterilizer  (§  245)  and  keep  the  water  boiling  for  a  half  hour.  Pro- 
tect the  tubes  from  dust  until  needed. 

Any  clear  soup  or  bouillon  will  serve  for  the  following  experiment. 
The  clear  concentrated  soups  commonly  sold  at  ten  cents  per  can 
may  be  used  after  diluting  with  water.  Test  with  litmus-paper,  and 
if  acid  add  some  baking  soda  (or  KOH  solution) ,  little  by  little,  until 
the  soup  is  slightly  alkaline  in  reaction.  When  thus  prepared,  the 
bouillon  may  be  kept  in  a  bottle  or  flask  by  simply  plugging  the 
mouth  with  cotton  and  sterilizing  after  each  opening  of  the  bottle. 

Fill  sterile  test-tubes  half  full  with  the  bouillon;  and  replace  the 
cotton  plugs,  taking  care  that  they  do  not  become  wet.  Place  the 
tubes  in  the  sterilizer  for  a  half  hour,  and  repeat  on  the  following 
day.  The  sterile  tubes  may  now  be  kept  indefinitely.  It  is  best  to 
keep  them  covered  so  that  dust  from  the  air  may  not  fall  on  the 
cotton  and  thus  increase  the  chance  of  bacteria  getting  into  the  tube 
when  the  plug  is  removed. 

Examine  tubes  which  have  remained  sterile  for  many  days.  If 
bacteria  develop,  the  bouillon  will  become  turbid  or  cloudy. 

Take  the  plugs  from  some  tubes  with  bouillon,  and  leave  the  tubes 
open  for  several  days.  Do  bacteria  develop?  What  is  the  ex- 
planation of  the  fact  that  similar  tubes  with  cotton  plugs  remained 
sterile  ?  The  cotton  certainly  cannot  keep  air  out  of  tubes ;  what 
then  can  be  its  effect  on  the  air  which  enters  the  tubes  ? 

Mount  a  drop  of  bouillon  from  a  tube  which  has  become  turbid. 
Examine  with  a  high  power.  It  is  well  to  have  some  cotton  threads 
under  the  cover-glass  to  assist  in  finding  the  focus.  Examine  with 
a  high  power.  Look  for  exceedingly  minute,  transparent  bodies, 
much  smaller  than  yeast  cells  and  mold  spores ;  some  rod-shaped, 
some  spherical,  some  twisted  rods  (Fig.  86).  By  shifting  the  mirror 
of  the  microscope,  it  is  possjble  to  see  them  better.  Some  of  these 
organisms  (bacteria)  swim  rapidly  by  means  of  cilia  so  small  that 
they  cannot  be  seen  with  the  ordinary  microscope.  Watch  the 
swimming  of  some  of  the  largest  bacteria  visible.  Remember  that 
the  apparent  rate  of  speed  has  been  magnified  as  much  as  have 


STUDIES   OF  SPORE-PLANTS  279 

the  bacteria,  and  that  the  distance  apparently  covered  must  be 
divided  by  the  magnification  of  the  microscope  used  in  order  to 
learn  the  actual  speed.  Glass  slides  with  delicate  ruled  lines  have 
been  used  to  estimate  the  size  and  speed  of  the  bacteria.  Some  of 
those  often  seen  are  so  small  that  50,000  placed  end  to  end  would 
make  a  row  one  inch  long,  and  a  bacterium  rssinr  of  an  inch  long  be- 
longs to  a  giant  species. 

In  order  to  make  bacteria  more  distinctly  visible,  it  is  customary 
to  stain  them.  A  simple  method  is  as  follows.  Place  a  drop  of 
fluid  with  bacteria  on  a  cover- 
glass,  evaporate  the  water 
slowly  by  holding  the  glass 
above  a  gas-burner  (a  special 
kind  of  forceps  is  made  for 
holding,  but  ordinary  forceps 
will  serve),  then  pour  over  the 
dry  bacteria  a  few  drops  of  FIG.  86.  Forms  of  bacteria.  1,  micro- 
stain  (gentian-violet,  or  other  cocci.  some  in  chains  (streptococci), 
aniline  dyes  dissolved  in  ?•  *******  ^°  flagellated  3  bacilli 
v  .  I.  in  chains.  4,  spirilla.  5,  bacilli  with 

water).    After  twenty  or  more        8poreg      ^  a  dividing   bacillus  with 

minutes,  wash  off  the  stain  by        flagella. 
dipping  the  slide  into  clean 

water,  and  then  place  the  cover-glass  on  a  drop  of  water  on  an  object- 
slide  for  microscopic  examination.  Be  careful  to  get  the  stained  side 
of  the  cover-glass  down.  Instead  of  mounting  with  water,  the  cover- 
glass  may  be  carefully  dried  after  washing  off  the  stain,  and  mounted 
with  a  drop  of  glycerin  or  Canada  balsam.  The  glycerin  will  preserve 
the  preparation  for  several  days,  and  the  balsam  makes  a  permanent 
preparation  which  may  be  kept  for  years. 

Make  brief  examination  of  bacteria  obtained  from  various  sources, 
such  as  sour  milk,  "mother-of-vinegar,"  and  any  organic  materials 
found  in  a  state  of  decomposition.  All  bacteria  seen  will  be  spheri- 
cal, rod-like,  twisted,  or  spiral  in  shape,  but  they  may  be  found 
united  together  in  groups. 

Bacteria  may  be  found  which  show  an  enlargement  due  to  the 
formation  of  a  spore  (Fig.  86).  These  spores  appear  as  dark  spots 
when  the  microscope  is  slightly  out  of  focus,  and  very  glistening 
when  in  focus.  "  Hay-tea  "  made  by  pouring  hot  water  over  some 
good  hay,  and  filtering  through  a  layer  of  cheesecloth,  is  excellent  for 
growing  bacteria  for  study  of  spores.  Fill  several  test-tubes  half 
full  of  "hay-tea, "  plug  with  cotton,  heat  to  100°  C.  by  holding  tubes 
in  gas-flame  until  boiling  occurs,  or  for  a  short  time  in  a  sterilizer. 


280  APPLIED  BIOLOGY 

Sterilize  half  of  the  tubes  again  on  second  and  third  days.  The 
tubes  sterilized  once  will  probably  become  turbid  within  a  few  days 
and  microscopic  examination  will  show  that  practically  all  the  con- 
tained bacteria  are  rod-like.  Later  they  will  form  spores  ;  and  this 
is  the  secret  of  the  bacteria  appearing  after  once  boiling.  The  spores 
were  able  to  withstand  the  heat ;  but  before  the  second  or  the  third 
heating  they  had  germinated  and  the  bacteria  thus  formed  were 
killed  by  the  later  heating. 

Pure  Cultures.  —  It  often  happens  in  the  above  experiment  that 
the  tube  heated  once  has  practically  a  pure  culture  of  a  certain  kind 
of  bacteria.  This  is  due  to  the  fact  that  all  active  bacteria  are 
killed,  and  when  the  spores  germinate  the  conditions  (food,  acidity) 
are  best  suited  to  the  one  kind  of  bacteria  which  flourishes.  Even 
if  other  kinds  were  suited  to  the  existing  conditions,  the  form  which 
is  most  abundant  first  may  gradually  crowd  out  the  other  kinds. 

Now,  this  is  one  way  of  getting  approximately  pure  cultures  of 
certain  bacteria;  but  for  more  accurate  work  and  for  separating 
many  species  which  live  together,  it  is  necessary  to  have  a  method 
for  isolation  of  individual  bacteria.  This  is  afforded  by  Koch's 
culture-plate  method,  which  uses  a  solid  medium  like  gelatin  instead 
of  a  liquid  medium  like  bouillon.  In  a  liquid  the  bacteria  are  con- 
stantly moving,  and  hence  mixing ;  but  on  a  gelatin  plate  a  bacterium 
is  fixed  at  a  certain  spot,  and  by  division  large  colonies  of  its  de- 
scendants are  developed. 

Gelatin  Plates.  —  (D)  Run  a  thin  layer  of  some  melted  gelatin 
medium  (gelatin  melted  in  bouillon ;  see  directions  in  ' '  Teachers ' 
Manual")  into  the  bottom  part  of  some  Petri  dishes,  cover,  and 
sterilize  for  a  half -hour.  Repeat  sterilization  on  second  and  third  days. 
Take  care  to  keep  the  dishes  level  so  that  the  melted  gelatin  does 
not  touch  and  glue  the  cover  fast.  After  the  last  sterilizing,  allow 
the  gelatin  to  cool  so  as  to  form  a  plate  in  the  bottom  of  the  Petri 
dish.  Wide-mouthed  bottles,  stoppered  with  cotton,  can  be  used 
in  same  way,  simply  adding  enough  gelatin  to  form  a  thin  plate 
on  the  side  (Fig.  87).  Small  bottles  and  test-tubes  may  be  used,  but 
they  should  be  left  inclined  after  the  last  sterilizing,  so  that  the  cooled 
gelatin  will  be  an  inclined  plate  against  a  side  of  the  tube  or  bottle. 

(D)  Select  some  Petri  dishes  in  which  the  gelatin  plates  have 
remained  sterile  for  several  days.  Remove  the  covers,  and  leave 
the  gelatin  plate  exposed  to  the  air  of  the  room  for  five,  ten,  or 
twenty  minutes.  Then  replace  covers,  and  label.  Look  at  the 
dishes  from  day  to  day.  If  spots  (white,  yellow,  or  red)  appear, 
notice  that  they  increase  in  size  from  day  to  day.  Each  spot  is 


STUDIES  OF  SPORE-PLANTS  281 

usually  made  up  of  the  descendants  of  one  bacterium  which  happened 
to  fall  on  the  gelatin.  Of  course,  two  kinds  of  bacteria  might 
happen  to  be  on  the  same  dust  particle  which  falls  on  the  plate  of 
gelatin  exposed  to  the  air ;  but  microscopic  examination  of  some  of 
the  bacteria  removed  by  a  sterile  needle  would  disclose  that  fact, 
and  it  is  easy  to  select  spots  which  are  pure  cultures  ;  that  is,  made 
up  of  one  kind  of  bacteria. 

Now,  if  a  sterilized  inoculating  needle  be  touched  to  one  of  these 
spots  and  then  stroked  across  the  gelatin  on  a  new  sterile  plate,  or 


FIG.  87.     A  flat  bottle  may  be  used  in  place  of  a  Petri  dish   for  gelatin 
plates.     The  mouth  is  stoppered  with  cotton.     (From  Osterhout.) 

in  test-tubes,  the  bacteria  transferred  to  the  new  plate  will  probably 
be  all  of  one  kind,  and  when  they  multiply  a  pure  culture  will  be 
obtained. 

When  once  a  pure  culture  is  obtained,  sub-cultures  to  new  plates, 
or  to  bouillon  or  gelatin  in  tubes,  are  easily  made  with  a  sterile  in- 
oculating needle  (§  245).  Great  care  must  be  taken  when  opening 
plates  and  tubes,  in  order  to  avoid  dust  which  may  contaminate  the 
cultures  with  other  bacteria.  In  research  work  with  bacteria,  special 
glass  cases  are  often  used  for  keeping  dust  from  tubes  while  transfers 
of  bacteria  in  pure  cultures  are  being  made. 

In  case  it  is  desired  to  separate  the  bacteria  in  a  liquid,  as  in  water 
analysis,  one  or  more  drops  of  the  liquid  are  poured  on  a  sterile 
gelatin  plate,  and  the  liquid  is  allowed  to  run  across  the  plate.  The 
contained  bacteria  will  start  colonies  along  the  path  of  the  drop,  and 
after  selecting  colonies  to  be  cultivated,  some  bacteria  from  them 
are  transferred  to  new  sterile  plates.  Allow  a  drop  of  water  from 
a  hydrant  to  flow  across  sterile  gelatin  in  a  Petri  dish,  cover 
quickly  and  examine  next  day.  Sometimes  the  number  of  colonies 
of  bacteria  will  be  so  great  that  it  is  impossible  to  select  a  colony 
that  is  pure.  In  such  a  ease  the  drop  of  water  to  be  examined  must 
be  mixed  with  many  drops  of  absolutely  sterile  water  (distilled  or 
boiled)  in  order  to  scatter  the  contained  bacteria,  and  a  drop  from 
this  run  on  a  gelatin  plate  will  give  fewer  colonies. 


282  APPLIED  BIOLOGY 

In  similar  ways  bacteria  from,  any  source  may  be  obtained  in  pure 
cultures  by  means  of  solid  culture  media.  In  addition  to  gelatin,  a 
thin  slice  of  potato,  carefully  sterilized,  may  be  used  ;  and  for  certain 
bacteria  which  must  be  grown  in  incubators  agar-agar  is  added, 
because  it  is  not  so  easily  melted  as  gelatin. 

256.  Important  Facts  concerning  Structure  and  Functions 
of  Bacteria.  —  (a)  Size.  The  average  rod-shaped  bacteria 
are  not  more  than  -nrFoir  °f  an  incn  m  length  and  ^riinr  in 
diameter.  Many  of  the  spherical  forms  are  not  more  than 
•STj-tfu-^  of  an  inch  in  diameter;  some  are  still  smaller  and  just 
visible  with  the  highest  powers  of  the  microscope,  and  the 
germs  (supposed  to  be  bacteria)  of  the  foot-  and-mouth  disease 
of  cattle  are  so  small  that  they  will  pass  through  the  pores  of 
the  earthenware  tubes  in  a  Pasteur-Berkefeld  filter,  which 
will  remove  all  bacteria  large  enough  to  be  seen  with  the 
microscope.  In  fact,  bacteria  are  so  small  that  fractions  of 
inches  are  inconvenient  measures;  and  biologists  commonly 
state  dimensions  in  micromillimeters  or  microns,  one  of  which 
is  nnnr  °f  a  millimeter,  or  about  ^-grinr  of  an  inch.  A  bac- 
terium OTTO-  mch  long  would  be  1  micron  long. 

(b)  Growth  and  Division.  Under  favorable  conditions,  a 
bacterium  rapidly  grows  to  full  size  and  then  divides  into 
equal  halves.  Each  of  these  absorbs  food,  grows,  and  soon 
divides  again.  Certain  bacteria  have  been  observed  to 
divide  every  twenty  minutes.  Some  one  has  computed  that 
if  the  descendants  of  a  single  individual  bacterium  were  to 
keep  on  dividing  once  an  hour  for  two  days  there  would  be 
more  than  280  millions.  You  can  easily  verify  these  figures 
by  applying  the  mathematical  formulas  for  geometrical  pro- 
gression. At  the  rate  of  one  division  every  half  hour,  there 
would  be  4772  billions  in  72  hours,  and  the  weight  would 
probably  be  more  than  7000  tons.  These  figures  simply 
illustrate  the  enormous  reproductive  powers  of  the  bacteria 
and  help  us  to  understand  how  a  few  bacteria  introduced 
may  soon  produce  enough  to  cause  disease  or  the  decompo- 


STUDIES   OF  SPOKE-PLANTS  283 

sition  of  foods.  But  the  truth  is  that  the  half-hour  or  hour 
rate  of  division  is  never  maintained  for  long  at  a  time.  Soon 
food-supply  is  exhausted,  their  own  excretions  accumulate 
and  exert  a  poisonous  influence,  or  other  unfavorable  condi- 
tions tend  to  check  the  rapid  rate  of  growth. 

(c)  Spore-formation  occurs  in  some  bacteria.     Only  a  few 
human  diseases,  and  those  not  very  common,  are  caused  by 
bacteria  which  produce  spores.     This  is  a  fortunate  circum- 
stance, for  as  we  have  seen,  spores  are  much  more  difficult  to 
kill  than  are  the  active  bacteria.     As  a  rule,  one  bacterium 
produces  only  one  spore,  and  this  germinates  and  forms  one 
bacterium.     There  is  therefore  no  multiplication,  and  spore- 
formation  is  simply  a  device  for  adapting  the  organism  to 
unfavorable  conditions.     Spores  taken  from  cattle  dead  from 
the  anthrax  disease  have  been  found  capable  of  germinating 
many  years  after  the  animals  were  buried. 

(d)  Temperature.     Some   bacteria   are   able   to   multiply 
near  the  freezing  point,  and  some  live  in  hot  springs  in  water 
at  a  temperature  which  will  kill  most  kinds  of  bacteria  and 
other  living  things.     Between  these  two  extremes  there  are 
all  gradations.     Contrary  to  the  popular  belief,  freezing  does 
not  kill  all  bacteria.     Bacteria  of  several  species,  including 
those  which  cause  typhoid  and  diphtheria,  have  been  kept  sev- 
eral days  at  the  temperature  of  liquid  air  (about  —  190°  C.), 
and   when   thawed   out,    appeared   to   multiply   normally. 
However,  a  very  large  percentage  of  common  bacteria  die 
when  frozen  in  ice,  and  comparatively  few  are  living  after 
the  ice  has  been  kept  five  or  six  months.     A  few  years  ago  an 
epidemic  of  typhoid  fever  was  traced  to  ice  which  had  been 
stored  seven  months,  so  that  all  ice  from  waters  contaminated 
with  sewage  should  be  regarded  with  suspicion.     While  most 
of  the  bacteria  will  probably  die,  the  few  which  remain  may 
multiply  rapidly  when  taken  into  the  human  body. 

The  thermal  death-point  varies.     Ten  minutes'  exposure 
to  a  temperature  of  70°  C.,  or  one  minute  at  100°  C.  (boil- 


284  APPLIED   BIOLOGY 

ing),  will  kill  all  bacteria  of  typhoid  and  tuberculosis.  Spores 
of  other  bacteria  may  survive  the  boiling  point  for  hours; 
but  boiling  for  a  short  time  on  successive  days  (known  as 
discontinuous  sterilization)  will  cause  the  spores  to  germi- 
nate, and  then  they  are  easily  killed  by  heat.  Fortunately, 
the  disease-producing  (pathogenic}  bacteria  likely  to  be  in 
drinking  water  and  milk  do  not  form  spores. 

Sterilization  of  substances  containing  bacteria  is  usually 
accomplished,  as  already  described  for  molds  (§  245),  by  dis- 
continuous steaming  or  boiling  on  two  or  three  days.  The 
first  boiling  (100°  C.)  swells  the  spores,  and  the  later  heating 
kills  them.  Steam  under  about  30  Ib.  pressure  and  having 
a  temperature  of  about  130°  C.  will  kill  even  resistant  spores 
in  half  an  hour.  Dry  hot  air  at  about  350°  C.  is  also  effi- 
cient as  a  sterilizer  of  such  articles  as  clothing. 

Pasteurization  (discovered  by  Pasteur)  means  heating 
milk  to  a  temperature  of  60  to  70°  C.,  and  keeping  at  this 
temperature  for  10  to  20  minutes.  Its  advantage  is  that  it 
does  not  change  milk  as  does  heating  to  boiling  point  (100°  C.), 
while  it  kills  the  disease  bacteria  likely  to  be  present  in  im- 
pure milk.  In  some  cities  much  milk  is  now  pasteurized. 

(e)  Light.  —  Strong  sunlight  kills  all  bacteria  which  are 
directly  exposed,  as  on  the  surface  of  soil.  Some  species  are 
killed  in  a  few  minutes,  and  few  can  withstand  hours  of 
exposure.  This  is  important,  for  it  suggests  the  value  of  sun- 
light for  killing  bacteria  in  sunny  rooms,  and  especially  on 
clothing,  carpets,  etc.,  which  can  be  exposed  out-of-doors. 
It  also  suggests  the  importance  of  building  houses  so  as  to 
get  the  maximum  of  sunlight ;  for  example,  shade  trees  should 
not  stand  too  near  houses.  The  value  of  sunlight  in  disin- 
fecting streets  cannot  be  too  strongly  emphasized.  Sprin- 
kling with  water  increases  the  action,  for  wet  bacteria  stand 
less  light.  It  is  unfortunate  that  all  streets  in  cities  cannot 
be  in  the  north-south  direction  so  that  all  their  surfaces  will 
get  the  full  force  of  sunlight  at  mid-day. 


STUDIES   OF  SPORE-PLANTS  285 

(/)  Chemicals.  —  Many  chemicals  kill  bacteria,  and  are 
called  germicides  or  disinfectants.  Some  chemicals  when 
much  diluted  do  not  kill  bacteria,  but  prevent  their  multi- 
plication. Such  are  called  antiseptics.  Corrosive  sublimate 
(mercuric  chloride),  and  carbolic  acid  are  two  powerful 
germicides,  killing  bacteria  even  when  much  diluted.  Most 
strong  acids  and  alkalies  are  also  germicides.  Chloride  of 
lime  and  washing  powders  are  two  efficient  germicides  for 
general  household  cleaning.  Among  gases,  sulphur  gas  from 
burning  sulphur  and  formaldehyde  from  evaporated  formalin 
are  now  regarded  as  most  powerful  germicides.  The  forma- 
lin is  better  because  it  does  not  bleach  colored  articles,  is  not 
so  poisonous  to  human  lungs,  and  is  easily  evaporated  in  a 
room  either  by  a  special  lamp  or  by  the  heat  generated  by  a 
bucket  of  slaking  lime  on  which  the  formalin  is  quickly 
poured  before  closing  the  door  of  the  room.  By  exposing 
cultures  of  bacteria  in  such  rooms,  it  has  been  shown  that 
the  bacteria  are  killed  by  the  gas  generated.  Among  common 
antiseptics  are  very  dilute  solutions  of  formalin,  carbolic 
acid,  boric  acid,  common  salt,  and  many  substances  pro- 
duced by  plants  (menthol,  thymol,  eucalyptol,  camphor, 
cloves,  cinnamon,  etc.).  The  preservative  action  on  foods 
of  common  salt,  vinegar,  creosote  (on  smoked  meats),  and 
spices  is  due  to  the  antiseptic  power  of  these  substances. 
The  more  powerful  preservatives,  such  as  boric  acid,  so- 
dium benzoate,  formalin,  and  salicylic  acid  are  sometimes 
used  in  foods ;  but  there  is  danger  of  injuring  the  digestive 
organs  by  amounts  so  small  that  the  sense  of  taste  does  not 
guard  against  them.  There  is  no  such  danger  with  vinegar 
and  common  salt. 

257.  Where  Bacteria  are  Found.  —  They  may  be  said  to 
be  almost  ubiquitous.  They  are  abundant  in  soils ;  water  of 
seas,  rivers,  and  lakes ;  in  the  bodies  of  animals  and  plants ; 
in  all  dead  organic  matter  in  nature ;  and  in  the  air  (except 
on  high  mountains,  in  polar  regions,  and  in  uninhabited  desert 


286  APPLIED  BIOLOGY 

areas  of  such  immense  size  that  bacteria  could  not  be  carried 
on  dust  from  other  regions).  In  fact,  bacteria  are  distrib- 
uted wherever  they  can  get  food  and  proper  temperature  for 
growth  and  multiplication.  Unlike  other  organisms,  cer- 
tain bacteria  do  not  require  oxygen  from  the  air,  for  they  can 
get  it  by  decomposing  the  organic  substances  on  which  they 
live.  Such  bacteria  are  called  anaerobic  (living  without  air). 
Some  of  them  can  live  deep  in  the  soil ;  but  at  depths  below 
where  roots  of  plants  and  earthworms  penetrate,  the  soil  is 
usually  without  bacteria.  Hence  water  from  deep  wells  is 
usually  pure,  unless  there  are  openings  between  strata  which 
somewhere  communicate  with  lakes  or  other  surface  water. 

The  wide  distribution  of  bacteria,  especially  on  or  in  every- 
thing connected  with  the  inhabited  surface  of  the  earth, 
makes  it  extremely  difficult  to  eliminate  them.  In  fact, 
only  in  closed  bottles,  etc.,  in  which  all  bacteria  can  be  killed 
by  heat  or  chemicals,  is  it  possible  to  keep  any  substance 
free  from  bacteria  (i.e.,  sterile).  This  is  the  reason  for  the 
careful  work  necessary  in  preserving  many  foods. 

258.  Useful  Bacteria.  —  The  popular  impression  is  that  all 
bacteria  are  harmful  in  that  they  decompose  our  foods  or 
cause  dangerous  diseases  of  man  or  of  useful  animals.  This 
is  far  from  true,  for  very  many  bacteria  are  directly  useful. 

(a)  Bacteria  in  Soils.  —  Most  important  of  the  useful  bac- 
teria are  those  which  deal  with  the  nitrogen  compounds  in  the 
soil.  We  have  already  noted  that  the  nitrogen  excretions  of 
animals  are  in  turn  used  by  plants  (§  117) ;  but  these  excre- 
tions must  first  be  prepared  by  bacteria.  These  organisms 
first  decompose  the  rather  complex  animal  excretions  like  urea, 
and  then  unite  the  contained  nitrogen  into  simple  compounds 
(nitrites  and  nitrates)  which  the  roots  of  plants  can  absorb 
and  use.  The  guano  from  Peru,  which  has  long  been  a  valu- 
able agricultural  fertilizer,  was  formed  by  bacterial  action  on 
the  excreta  of  countless  thousands  of  sea-birds  which  lived 
in  prehistoric  times.  It  contains  concentrated  nitrogen  com- 


STUDIES  OF  SPORE-PLANTS  287 

pounds  readily  available  for  absorption  by  roots  of  plants. 
Nitrate  of  soda  from  Chile  was  probably  formed  by  bacterial 
action  which  united  the  nitrogen  from  organic  matter  (sea- 
weed, or  guano)  with  sodium  derived  from  sea-water.  Simi- 
lar bacterial  changes  take  place  in  all  manures  used  on  farms. 
The  odor  of  ammonia  around  stables  is  evidence  that  the 
decomposing  bacteria  are  at  work,  and  that  valuable  nitro- 
gen in  the  ammonia  is  escaping  into  the  air,  because  other 
kinds  of  bacteria  are  not  working  properly  and  fixing  the 
nitrogen  in  nitrites  and  nitrates.  Agricultural  books  give 
rules  for  handling  manures  so  as  to  avoid  such  wasteful 
decomposition ;  that  is,  they  teach  how  to  cause  both  kinds  of 
bacteria  to  work  together  so  that  the  products  of  decompo- 
sition will  be  built  into  available  crude  plant  foods  with  little 
loss  of  nitrogen  (e.g.,  in  ammonia)  to  the  air.  This  is  a 
problem  of  such  great  financial  value  that  every  farmer 
should  study  it  carefully  in  the  special  books  on  soil  fer- 
tilizers. 

In  still  another  way  certain  bacteria  are  useful  in  soils. 
The  free  nitrogen  in  the  air  is  not  usable  by  animals  and 
plants,  with  the  sole  exception  of  certain  bacteria  known  as 
the  nitrogen-fixing  bacteria,  which  live  in  the  root-tubercles 
on  certain  plants  (§  152).  These  bacteria  unite  the  nitrogen 
absorbed  from  the  air  into  nitrogen  compounds  which  can 
be  used  by  the  plants  that  have  the  tubercles.  In  recent 
years,  the  importance  of  this  method  of  increasing  the  amount 
of  nitrogen  fertilizers  in  the  soil  has  been  recognized,  and 
now  every  scientific  farmer  gives  special  attention  to  raising 
clovers,  alfalfa,  peas,  vetches  and  similar  plants  whose  roots 
are  favorable  to  the  nitrogen-fixing  bacteria.  By  growing 
such  plants  soils  can  be  much  improved  in  fertility  at  an 
expense  much  lower  than  by  use  of  nitrate  of  soda  and  other 
commercial  fertilizers.  The  only  other  known  way  of  pre- 
paring nitrogen  from  the  air  for  use  by  plants  is  an  expen- 
sive method  of  combining  the  nitrogen  with  sodium  by 


288  APPLIED  BIOLOGY 

means  of  electrical  action.  So  far  the  nitrate  fertilizers  pro- 
duced electrically  are  even  more  costly  than  the  nitrate  of 
soda  from  Chile. 

(6)  Bacteria  in  Milk.  —  A  number  of  kinds  of  bacteria 
are  useful  in  milk,  butter,  and  cheese.  The  most  common 
bacteria  in  milk  are  those  which  produce  lactic  acid,  which 
causes  milk  to  sour  and  coagulate.  While  this  process  may 
be  undesirable  in  milk  intended  for  drinking,  it  is  useful 
in  the  making  of  certain  kinds  of  cheese.  Moreover,  the 
presence  of  the  lactic  bacteria  in  milk  prevents  the  growth  of 
certain  species  which  cause  putrefaction.  If  milk  be  heated 
to  70  or  75°  C.  (pasteurized),  the  lactic  bacteria  are  killed 
and  others  capable  of  producing  putrefaction  remain.  This 
fact  leads  to  the  belief  that  lactic  bacteria  are  useful  after 
milk  is  taken  into  the  stomach  in  that  they  oppose  other 
bacteria  which  may  be  harmful.  There  is  no  reason  to 
think  that  the  lactic  bacteria  in  milk  are  harmful  when 
taken  into  the  digestive  organs ;  but,  of  course,  most  people 
do  not  like  to  drink  sour  milk.  In  recent  years,  we  have  had 
many  newspaper  articles  on  Professor  Metchnikoff's  pre- 
pared milk  for  curing  and  preventing  disease  caused  by 
excessive  development  of  decomposing  bacteria  in  the  in- 
testines. Such  bacteria  produce  poisonous  substances  which 
may  be  absorbed  by  the  blood  and  cause  ill  health  (auto- 
intoxication, meaning  self-poisoning).  The  milk  prepared 
by  the  Metchnikoff  method  is  believed  to  contain  substances 
secreted  by  certain  bacteria  whch  will  prevent  excessive 
multiplication  of  putrefying  bacteria  in  the  intestine. 
These  products  of  bacteria  are  sometimes  concentrated  into 
tablets  to  be  taken  like  pills  or  dissolved  in  milk  before 
drinking;  but  most  of  the  tablets  on  the  market  are  abso- 
lutely valueless  in  physiological  effect.  In  fact,  physicians 
are  not  yet  agreed  as  to  the  medicinal  value  of  such  prepared 
milk ;  but  one  who  wishes  to  try  it  should  purchase  the  milk 
in  which  the  proper  bacteria  have  been  grown,  for  it  seems 


STUDIES   OF  SPOBE-PLANTS  289 

probable  that  only  in  this  way  can  their  secretions  be  abun- 
dant enough  to  check  other  bacteria  in  the  intestines. 

(c)  Bacteria  in  Butter  and  Cheese.  —  The  peculiar  flavors  of 
butter  and  cheese  are  due  to  certain  species  of  bacteria,  and 
hence  the  scientific  dairyman  gives  special  attention  to 
methods  which  will  prevent  the  growth  of  undesirable  species 
and  favor  the  desirable  ones.  By  using  pure  cultures  of 
bacteria  (and  sometimes  certain  molds),  it  is  possible  to  pro- 
duce the  desired  quality  of  butter  and  cheese.  The  dis- 
agreeable flavor  of  butter  made  in  many  farm-houses  is  due 
to  undesirable  bacteria.  See  Conn's  "  Bacteria,  Yeasts, 
and  Molds  " ;  and  pamphlets  of  the  United  States  Depart- 
ment of  Agriculture  on  butter  and  cheese  making. 

(d)  Bacteria  in  Vinegar.  —  The  change  of  alcohol  into 
vinegar  by  bacteria  has  been  mentioned  in  connection  with 
yeast.  "  Mother-of- vinegar  "  is  a  mass  of  bacteria  imbedded 
in  a  gelatinous  substance  which  they  secrete. 

(e)  Bacteria  and  Textile  Fibers.  —  Many  fibers,  such  as  those 
of  flax,  hemp,  and  others  obtained  from  the  stems  of  plants, 
are  obtained  by  rotting  the  stems  until  the  surrounding 
tissues  are  soft.  Bacteria  do  this  work. 

(/)  Useful  Decomposition.  —  While  the  action  of  bacteria 
in  decomposing  foods  prepared  for  human  use  is  harmful 
and  financially  a  source  of  great  loss,  we  should  not  forget 
that  this  same  process  of  decomposition  is  absolutely  neces- 
sary in  order  to  keep  up  the  cycle  of  organic  matter  (§§  116, 
117).  We  repeat  for  emphasis  the  statement  made  elsewhere 
that  but  for  the  decomposing  action  of  bacteria,  aided  by 
other  micro-organisms,  the  world  would  soon  be  so  filled 
with  dead  bodies  of  animals  and  plants  that  the  carbon  and 
nitrogen  available  in  the  food-supply  of  living  things  would 
be  bound  up  in  dead  organic  matter.  Hence  as  decomposers 
of  organic  jnatter  the  bacteria  are  indispensable. 

A  splendid  example  of  the  usefulness  of  bacteria  in  causing 
decomposition  is  found  in  the  latest  methods  for  disposal  of 


290  APPLIED  BIOLOGY 

sewage  by  distributing  it  in  concrete  tanks  or  reservoirs, 
arranged  so  as  to  allow  decomposing  bacteria  to  grow  rapidly. 
The  result  is  that  sewage  is  changed  so  that  it  will  be  harm- 
less. Even  pathogenic  (disease-producing)  bacteria  are  thus 
killed.  Many  cities  now  purify  their  sewage  by  this  method 
before  discharging  it  into  rivers.  For  private  sewer  systems 
in  villages  and  in  rural  districts,  the  best  method  consists 
in  allowing  sewage  to  run  into  porous  drain-tiles  laid  a  short 
distance  below  the  surface  in  loose  soil.  In  the  soil  around 
the  tiles  the  decomposing  bacteria  flourish  and  convert  the 
sewage  into  harmless  substances  which  may  be  used  by  roots 
of  plants.  The  old-fasjhioned  cesspools  are  being  super- 
seded by  the  modern  bacterial  methods,  for  the  cesspools  do 
not  favor  rapid  growth  of  bacteria,  and  moreover  they  are 
often  deep  enough  to  allow  escape  of  dangerous  sewage  into 
subterranean  water  courses. 

Summarizing  the  useful  aspects  of  bacteria,  it  is  evident 
that  their  usefulness  Jar  exceeds  their  harmfulness.  A  few 
individual  organisms  may  die  from  bacterial  diseases,  but 
the  continuance  of  life  on  this  planet  depends  directly  upon 
the  decomposing  action  of  bacteria,  which  allows  a  cycle  of 
organic  matter  (§116).  And  quite  apart  from  this  all- 
important  aspect  of  our  relation  to  bacteria,  their  value  in 
the  other  ways  we  have  noted  above  far  outweighs  their 
harmfulness  as  producers  of  disease.  Moreover,  in  the  not 
distant  future,  when  civilized  people  will  carefully  apply  the 
already  well-known  biological  laws  (§§482^90),  so  that 
bacterial  diseases  will  be  kept  under  control,  the  usefulness 
of  bacteria  will  attract  more  popular  attention  than  at 
present,  when  the  average  citizen  thinks  of  "  germs  ;>  only  as 
producers  of  disease. 

Most  of  the  useful  aspects  of  bacteria  are  well  treated  in 
Lipmann's  "  Bacteria  in  Relation  to  Country  Life,"  and 
more  briefly  in  Conn's  "  Bacteria,  Yeasts,  and  Molds  in  the 
Home." 


STUDIES   OF  SPORE-PLANTS  291 

259.  Diseases  Caused  by  Bacteria.  —  We  are  so  familiar 
with  the  idea  that  many  diseases  are  caused  by  "  germs  "  or 
bacteria,  that  it  seems  scarcely  possible  that  less  than  thirty 
years  ago  no  disease  had  been  shown  to  result  from  the 
growth  of  bacteria  in  the  human   body.     It  was  in  1876 
that   Robert  Koch,  of  Berlin,  made   pure  cultures  of  rod- 
shaped   bacteria  found   in   the   blood  of    cattle  and  sheep 
suffering  from  anthrax,  or  splenic  fever,  and  gave  the  first 
proof  that  bacteria  caused  disease.     Six  years  later  (1882) 
Dr.  Koch  demonstrated  beyond  question  that  human  tuber- 
culosis is  caused  by  a  bacterium  which  he  named  Bacillus 
tuberculosis ;   and  in  1883  he  proved  that  the  dreaded  dis- 
ease, Asiatic  cholera,  is  caused  by  another  specific  bacterium. 
These  discoveries  aroused  the  interest  of  many  investigators, 
with  the  result  that  in  less  than  thirty  years  there  has  been 
built  up  the  new  science  of  bacteriology,  which  has  already 
been  of  inestimable   benefit  to  the  human  race  and  bids 
fair  to  lead  soon  to  absolute  control  of  some  of  the  most 
dangerous  diseases. 

To-day  the  list  of  diseases  known  to  be  caused  by  bacteria 
is  a  long  one  and  each  year  becomes  longer.  The  following 
are  some  of  the  best  known:  tuberculosis  of  any  organ, 
cholera,  diphtheria,  typhoid  fever,  blood-poisoning,  syphilis, 
pneumonia,  meningitis,  gonorrhea,  influenza,  tetanus,  lep- 
rosy, bubonic  plague,  and  numerous  others  of  rarer  occur- 
rence. These  are  all  infectious,  or  germ  diseases;  and  some 
of  them  are  also  contagious,  or  liable  to  be  transmitted  by 
contact  with  patients. 

260.  Diseases    Caused    by    other    Micro-organisms.  —  A 
number  of  diseases  are  now  known  to  be  caused  by  micro- 
organisms which  are  animals,  while  the  bacteria  belong  to 
the   plants.     These    animal   parasites   are   one-celled,    and 
belong  to  the  group  of  the  Protozoa  (§  273).     The  terrible 
African  disease  known  as  sleeping  sickness,  malaria,  and  a 
form  of  dysentery  are  the  best  known  human  diseases  cer- 


292  APPLIED  BIOLOGY 

tainly  due  to  animals  which  produce  effects  similar  to  those 
of  bacteria  in  other  diseases. 

261.  Infectious  Diseases  not  yet  Understood.  —  The  fol- 
lowing diseases  are  almost  certainly  due  to  micro-organisms 
which  are  readily  transmitted  from  one  person  to  another, 
thus  making  them  infectious;    but  the  organisms  have  not 
been  discovered.     The  diseases  of  unknown  causation  are 
yellow     fever,     hydrophobia,     smallpox,     whooping-cough, 
measles,  scarlet  fever,  and  mumps.     The  last  five  of  these 
infectious    diseases    are    highly    contagious.     In    all    these 
diseases  it  has  been  possible  for  medical  men  to  do  much 
towards  controlling  their  spread  by  working  on  the  assump- 
tion that  some  undiscovered  micro-organism  is  the  cause, 
and  that  therefore  cases  of  the  diseases  should  be  handled 
according  to  principles  based  on  diseases  known  to  be  caused 
by  bacteria.     For  example,   yellow  fever  has  been  shown 
to  be  transmitted  by  bites  of  certain  kinds  of  mosquitoes 
which  have  previously   sucked   blood  from   a  yellow-fever 
patient ;   and  this  has  suggested  the  desirability  of  applying 
the  rules  first  worked  out  for  malaria  when  it  was  discovered 
that  the  cause  is  microscopic  animals  which  are  injected  into 
human  blood  by  mosquito  bites.     The  rules  simply  require 
destruction  of  the  mosquitoes  (§  329),  and  preventing  them 
from  biting  healthy  persons  and  especially  those  sick  with 
malaria  or  yellow  fever.     Similarly,  in  dealing  with  smallpox, 
scarlet  fever,  mumps,  measles,  and  whooping-cough,  physi- 
cians  have   assumed   the   existence   of   some   undiscovered 
micro-organisms,  and   have  required   isolation  of  patients, 
quarantining  of  exposed  persons,  and  disinfection  of  rooms 
and  of  all  articles  on  which  bacteria  from  patients  might 
have  lodged. 

262.  How  Bacteria  cause  Disease.  —  We  may  take  diph- 
theria as  an  example  in  showing  how  bacteria  cause  disease. 
We  all  know  that  the  events  connected  with  this  dreaded 
disease  are  as  follows :   A  child  is  "  exposed  "  to  diphtheria, 


STUDIES  OF  SPORE-PLANTS 

perhaps  in  school  where  other  pupils  have  shown  signs  of  the 
disease,  and  after  a  number  of  days,  the  symptoms  of  the 
disease  may  appear ;  examination  of  the  throat  discloses 
certain  peculiar  spots,  and  microscopic  examination  of 
material  from  the  surface  of  these  spots  shows  countless 
thousands  of  bacteria  of  the  diphtheria  species.  The  reason 
why  the  disease  did  not  appear  immediately  after  exposure 
is  that  it  required  time  for  the  one  or  few  bacteria  which 
first  lodged  in  the  throat  to  multiply  and  form  a  colony. 

Toxins. — Now,  the  symptoms  of  diphtheria  are  not  simply 
those  of  a  sore  throat,  for  the  patient  is  evidently  affected  in 
many  other  organs.  This  seems  surprising,  for  the  bacteria 
are  not  so  widely  distributed ;  but  the  explanation  has  been 
found  in  the  fact  that  the  colony  of  bacteria  secrete  a  poison 
(toxin)  which  is  absorbed  by  the  blood  and  then  distributed 
widely  in  the  body.  Thus  bacteria  growing  in  one  organ 
(local  infection)  may  profoundly  affect  other  organs  in  the 
body. 

263.  Antitoxins.  —  If  the  vitality  of  the  patient  is  strong 
enough  to  endure  the  diphtheria  toxin  for  some  days,  the 
climax  of  the  disease  is  passed  and  convalescence  ensues. 
The  explanation  of  this  conquering  of  the  disease  is  that  the 
cells  of  the  patient's  body  have  gradually  secreted  a  substance 
which  counteracts  the  toxin;  this  is  an  antitoxin.  Those 
patients  whose  bodies  are  not  able  to  secrete  enough  anti- 
toxin succumb  to  the  disease. 

Every  one  who  reads  newspapers  must  have  learned  that 
doctors  now  treat  diphtheria  with  an  antitoxin  obtained 
from  horses.  The  explanation  is  as  follows.  A  great  many 
children  are  not  strong  enough  to  make  in  their  tissues  anti- 
toxin sufficient  to  overcome  the  poisons  of  the  diphtheria 
bacteria;  and  it  cannot  be  known  at  the  beginning  of  the 
disease  whether  the  patient  is  going  to  be  strong  enough. 
Hence,  it  is  desirable  to  give  some  artificial  aid.  No  drug 
has  been  found  to  do  this.  But  taking  advantage  of  the  fact 


294  APPLIED  BIOLOGY 

that  diphtheria  toxins  will  cause  the  horse  tissues  to  form 
antitoxin,  which  appears  in  the  blood,  physicians  now  inject 
into  the  blood  of  the  human  patient  some  antitoxin  from 
horse  blood,  and  this  saves  the  child  from  being  seriously  ill 
while  his  own  cells  develop  antitoxin.  Thus  it  is  possible  to 
put  into  the  blood  of  a  child  on  the  first  days  of  diphtheria 
more  antitoxin  than  the  child  might  develop  in  his  own  cells 
after  days  of  illness. 

Some  health  laboratories  keep  inoculated  horses  constantly 
so  as  to  have  a  supply  of  antitoxin  ready  for  use  of  doctors 
who  discover  cases  of  diphtheria.  The  toxins  injected  into 
the  horses  does  not  make  them  appear  sick.  The  with- 
drawing of  comparatively  small  quantities  of  blood  for 
extracting  the  antitoxin  is  done  by  an  instrument  which  does 
no  serious  injury ;  and  more  antitoxin  may  be  obtained  from 
the  same  horse  every  month. 

264.  Other  Diseases  are  Similar.  —  This  story  of  the  rela- 
tion of  bacteria  to  diphtheria  is  very  similar  to  that  of  many 
other  diseases  now  known  to  be  caused  by  bacteria.  The 
bacteria  enter  the  body,  multiply  and  form  toxins.  Then 
antitoxins,  or  other  opposing  substances,  appear,  counteract 
the  toxins,  and  the  patient  recovers.  In  only  a  few  cases 
has  it  been  possible  to  find  in  other  animals  antitoxins  which 
can  be  injected  into  the  human  tissues  to  cure  or  prevent 
human  diseases. 

Ant  toxins  and  other  antibacterial  substances  are  specific. 
For  example,  diphtheria  antitoxin  will  not  cure  or  prevent 
any  other  disease,  and  the  antitoxin  which  surgeons  use  after 
injuries  by  Fourth-of-July  pistols  is  obtained  from  animals 
into  whose  bodies  the  toxins  of  tetanus  or  lockjaw  bacteria 
have  been  injected,  causing  the  animal's  tissues  to  make 
tetanus  antitoxin. 

266.  Immunity.  —  One  of  the  most  interesting  things  con- 
nected with  bacterial  diseases  is  the  fact  that  some  people 
never  have  certain  diseases  even  when  often  exposed  to  infec- 


STUDIES  OF  SPORE-PLANTS  295 

tion ;  and  also  it  is  a  rule  that  one  is  not  likely  to  have  the 
same  diseases  a  second  time.  Furthermore,  adults  are  not 
likely  to  have  the  diseases  which  commonly  affect  children. 
This  lack  of  susceptibility  to  diseases  is  known  as  immunity. 
Immunity  which  is  present  in  human  or  animal  individuals 
who  are  not  susceptible  to  a  certain  disease  is  known  as  natural 
immunity,  while  that  which  follows  an  attack  of  a  disease 
is  said  to  be  acquired. 

Natural  immunity  is  much  more  common  than  is  suscep- 
tibility to  germ  diseases.  Probably  the  bacteria  cause 
disease  in  only  a  small  percentage  of  the  individuals  which 
they  enter.  It  is  known  that  germs  of  pneumonia  and  other 
diseases  are  often  present  in  persons  who  show  no  signs  of 
disease.  The  reason  why  we  do  not  develop  disease  every 
time  a  pathogenic  organism  enters  our  bodies  is  due  to 
(a)  destruction  of  bacteria  by  the  white  cells  of  the  blood 
and  lymph,  (6)  killing  of  bacteria  by  opposing  soluble  sub- 
stances in  the  blood,  (c)  prevention  of  growth  of  bacteria  by 
antiseptic  conditions  in  the  body,  and  (d)  counteracting  of 
toxins  of  the  bacteria  by  antitoxins  secreted  by  the  cells  of 
our  bodies.  The  relative  value  of  these  four  methods  of 
protection  varies  with  health  and  with  individuals.  In 
general,  all  four  are  most  efficient  when  there  is  good  health, 
and  hence  hygiene  by  improving  general  health  helps  the 
human  body  in  opposing  pathogenic  organisms.  A  good 
illustration  of  this  is  the  fact  that  building  up  the  general 
health  is  so  all-important  in  the  cure,  as  well  as  in  the  pre- 
vention, of  consumption  or  tuberculosis. 

Vaccination  is  an  example  of  acquired  immunity  produced 
without  an  attack  of  smallpox,  but  by  substitution  of  a  simi- 
lar and  harmless  disease  known  as  cow-pox.  ,In  fact,  cow- 
pox  seems  to  be  smallpox  which  has  been  weakened  by  devel- 
oping in  cows ;  and  so  when  vaccine  matter  is  taken  from  the 
pustules  on  cows  and  rubbed  into  a  cut  or  scratch  on  the 
human  skin,  the  result  is  a  mild  development  of  cow-pox. 


296  APPLIED  BIOLOGY 

This  causes  the  human  tissues  to  produce  some  opposing 
substance  which  effectually  prevents  the  disease  smallpox  for 
many  years,  the  length  of  immunity  varying  with  individuals. 

No  scientific  man  questions  that  vaccination  against 
smallpox  has  been  one  of  the  means  (isolation  and  disinfec- 
tion are  others)  which  has  made  smallpox  one  of  the  rarest 
diseases;  and  that  it  should  be  practiced  whenever,  in  a 
particular  locality,  there  are  cases  which  may  have  spread 
infection  widely.  For  example,  when  a  case  appears  in  a 
school,  all  teachers  and  pupils  in  that  school  should  be  imme- 
diately protected  by  vaccination  performed  by  competent 
doctors. 

Protective  Inoculation.  —  Similar  to  vaccination  is  protec- 
tively inoculating  by  giving  weakened  doses  of  toxins. 
Pasteur  discovered  that  if  the  bacteria  of  cattle  anthrax,  or 
those  of  chicken  cholera,  be  grown  in  pure  cultures  in  test- 
tubes,  the  toxins  get  weaker  as  the  cultures  grow  older.  In 
other  ways,  also,  it  is  possible  to  weaken  toxins  of  bacteria. 
Now,  if  weakened  toxins  are  injected  first,  there  is  a  mild 
attack  of  the  disease.  Then  a  stronger  toxin  will  produce 
no  more  effect ;  and  using  in  succession  in  a  series  of  days  or 
weeks,  stronger  and  stronger  toxins,  the  animal  into  which 
they  are  injected  finally  becomes  unable  to  take  the  disease 
and  is  said  to  be  protectively  inoculated.  Many  recent 
experiments  on  thousands  of  soldiers  seem  to  prove  that  it 
will  be  possible  to  inoculate  against  typhoid  in  a  similar  way. 
The  destructive  cholera  of  pigs  and  the  distemper  which 
annually  kills  thousands  of  dogs  are  being  studied  in  an 
attempt  to  find  a  successful  way  of  protectively  inoculating. 

Hydrophobia.  —  The  treatment  for  this  disease  is  another 
well-known  example  of  protective  inoculation.  Persons 
bitten  by  dogs  believed  to  have  rabies  go  to  a  Pasteur  Insti- 
tute, and  receive  frequent  injections  of  weakened  toxins 
obtained  from  the  spinal  cords  of  rabbits  which  have  died 
with  the  disease.  The  result  of  these  weakened  doses,  which 


STUDIES  OF  SPORE-PLANTS  297 

are  increased  in  strength  with  each  successive  injection,  is 
that  the  person  injected  acquires  immunity;  that  is,  he 
probably  develops  an  anti-toxin  faster  than  the  toxins  intro- 
duced by  the  saliva  of  the  rabid  dog  can  act.  At  present 
no  other  method  of  dealing  with  this  terrible  disease  is  known, 
and  hence  it  is  important  that  protective  inoculation  be 
applied  as  soon  as  possible.  But  much  common  sense  is 
needed  in  dealing  with  dog  bites.  There  is  no  foundation 
for  the  idea  that  dogs  are  liable  to  go  "  mad  "  in  "  dog  days." 
Most  dogs  which  bite  or  appear  sick  are  not  rabid.  Hydro- 
phobia is  a  rare  disease.  One  who  is  bitten  by  a  dog  should 
have  the  wound  properly  treated  by  a  surgeon,  for  blood- 
poisoning  bacteria  may  get  into  any  deep  wound.  If  the 
dog  shows  signs  of  illness,  it  should  be  kept  in  confinement 
for  some  days ;  for  if  it  is  rabid,  the  disease  will  develop 
rapidly.  If  the  dog  has  been  killed,  its  body  should  be 
expressed,  packed  in  ice,  to  the  nearest  Pasteur  Institute  or 
health  laboratory.  Microscopic  examination  of  the  nervous 
organs  will  show  whether  the  dog  had  hydrophobia,  and  it 
can  be  quickly  determined  whether  protective  inoculation 
should  be  given  to  the  person  who  was  bitten. 

References :  Limited  space  has  made  it  necessary  to  con- 
fine this  chapter  to  statements  of  general  principles  regard- 
ing bacteria.  Some  practical  notes  on  bacteria  and  human 
health  are  given  in  the  next  part  of  this  book  (§§  482-490). 
Sternberg's  "  Infection  and  Immunity  "  is  a  good  popular 
account  of  bacteria  and  disease.  Jordan's  "  General  Bac- 
teriology "  is  an  excellent  semi-technical  book. 


PART  III 

PRINCIPLES  OF  BIOLOGY  ILLUSTRATED  BY 
TYPES  OF  ANIMALS 

WE  have  seen  in  Part  II  how  types  of  plants  illustrate  the 
principles  of  biology  which  were  introduced  by  study  of  the 
bean  plant,  and  also  give  us  a  wider  acquaintance  with  rep- 
resentatives of  the  various  groups  of  plants  which,  while 
widely  different  in  form,  are  remarkably  similar  in  their 
fundamental  life-activities.  We  shall  now  make  a  series 
of  parallel  studies  on  the  animal  side  of,  biology.  In  study- 
ing plants  we  began  with  the  bean,  which  belongs  to  the 
highest  plants ;  and  then  in  succession  we  passed  down  the 
scale  of  plant  life  to  conifers,  ferns,  mosses,  algae,  fungi,  and 
the  simplest  plants.  In  our  study  of  types  of  animals  it  will 
be  interesting  to  reverse  this  order,  and  instead  of  going 
gradually  down  to  the  simplest  animals,  we  shall  pass  directly 
from  the  frog  to  the  one-celled  animals,  and  afterwards 
examine  a  number  of  types  of  animals  representing  the  gradu- 
ally increasing  complexity  of  animal  structure  up  to  the 
highest  vertebrates. 


299 


CHAPTER  X 
THE   SIMPLEST  ANIMALS:    THE   PROTOZOA 

266.  One-Celled  Animals.  —  The  simplest  animals  con- 
sist of  one  cell;  that  is,  they  consist  of  a  small  mass  of  pro- 
toplasm with  a  nucleus.     Within  this  one  cell  must  be  carried 
on  all  the  processes  connected  with  the  fundamental  life- 
activities    (feeding,   breathing,   excreting,   reproducing),   for 
each  of  which  processes  an  animal  like  a  frog  has  special 
organs  with  thousands  of  cells.     In  order  to  understand  how 
an  animal  with  one  cell  can  carry  on  the  same  life-activities 
as  does  an  animal  with  thousands  of  cells,  it  is  necessary  to 
study  some  examples  of  the  simplest  animals. 

267.  Paramecium.  —  (D  or  L)    If  some  chopped  hay  be  placed  in 
water  in  a  fruit-jar  or  other  convenient  vessel,  and  then  to  this  be 
added  some  decaying  sticks,  leaves,  and  other  objects  taken  from 
a  pond  where  aquatic  plants  are'growing,  there  will  probably  develop 
within  a  few  weeks  large  numbers  of  transparent  animals  appearing 
to  the  naked  eye  as  minute  whitish  specks.     With  a  rubber-bulbed 
pipette  take  a  drop  of  water  from  the  surface  and  near  the  edge  of 
the  vessel,  and  place  on  an  object-slide.     Then  place  a  few  shreds  of 
cotton  on  the  slide,  and  put  on  the  cover-glass. 

Place  the  slide  on  a  piece  of  black  cloth  or  paper,  and  notice  the 
moving  white  specks.  Use  a  hand-lens.  Now  examine  with  a  mi- 
croscope, using  first  the  low-power  objective.  Look  for  rapidly 
moving  objects  having  the  form  shown  in  Fig.  88,  A.  These  are 
specimens  of  paramecium,  sometimes  called  the  "slipper-animal- 
cule." In  size,  they  are  about  Tfo-  inch  long.  Several  other 
similar  animals  are  often  abundant  in  the  same  water. 

Study  the  following  points :  form  of  body,  movements,  response 
to  stimuli  (e.g.,  when  the  animals  bump  against  obstructions).  Is 
there  evidence  that  one  end  is  anterior  (i.e.,  goes  forward  in  loco- 
motion) ? 

300 


THE  SIMPLEST  ANIMALS 


301 


Mount  and  examine  another  slide  with  some  paramecia  which 
have  been  swimming  for  a  half-hour  in  carmine-water  (powdered 
carmine  mixed  with  water,  or  lamp-black  in  water).  The  particles 
of  carmine  appear  to  be  in  droplets  of  water  in  the  protoplasm  of  the 
animal.  Notice  the  action  of  the  lashing  cilia  upon  the  particles 


B 


C.TOC 


Inc.  07! 


c.vac. 


FIG.  88.  Paramecium.  A,  from  lower  surface.  B,  in  optical  section. 
c.vac,  contractile  vacuoles;  f.vac,  food  vacuole;  nu,  nucleus;  buc.gr,  buccal 
groove;  gul,  gullet;  mth,  mouth;  cu,  cuticle;  corf,  cortex,  outer  layer  of 
protoplasm;  med,  inner  layer.  (From  Parker.) 

of  carmine  surrounding  the  animal,  and  especially  the  collection  of 
particles  in  the  groove  on  one  side  of  the  body  and  then  their  en- 
trance into  the  body  through  a  tube  or  gullet  (see  arrows  in  Fig.  88, 
B).  This  taking  of  carmine  illustrates  how  the  animal  gets  food, 
chiefly  bacteria  and  other  small  plants  or  animals.  Notice  that 
there  is  no  stomach  or  other  cavity  into  which  food  is  taken,  but 
simply  particles  of  food  surrounded  by  a  film  of  water  are  forced 
into  the  protoplasm,  which  is  soft  and  semi-liquid.  These  food 


302  APPLIED  BIOLOGY 

masses  (called  food-vacuoles)  move  around  as  indicated  by  the 
arrows  in  the  figure;  digestive  juices  in  the  paramecium  slowly 
dissolve  the  digestible  food ;  the  protoplasm  absorbs  the  dissolved 
foods,  and  the  indigestible  remains  (such  as  hard  parts  of  some 
small  animals  and  plants  often  eaten,  or  the  carmine  particles  in 
above  experiment)  are  ejected  at  a  soft  spot  in  the  cell-membrane, 
as  shown  just  below  the  gullet  in  Fig.  88,  B. 

Using  the  high-power  objective,  examine  the  cilia  carefully. 
Look  for  the  two  clear  spots  shown  in  Fig.  88,  A  ;  they  are  spaces 
in  the  protoplasm  filled  with  water,  and  when  the  surrounding 
protoplasm  contracts  the  contents  are  ejected  through  a  little  canal 
leading  out  of  the  body.  They  are  excretory  organs,  known  as  con- 
tractile vacuoles  or  pulsating  vesicles,  and  the  water  they  eliminate 
contains  some  nitrogenous  excretions.  Sometimes  they  pulsate  as 
regularly  as  the  heart  beats  in  higher  animals. 

The  small  spindle-shaped  bodies  seen  near  the  cell-wall  in  the 
Figs.  88,  A  and  B,  contain  long  threads  which  are  thrown  out  when 
the  animal  is  irritated  by  chemicals  or  by  pressure.  Their  purpose 
is  believed  to  be  defense  against  other  lower  animals.  Some  of 
these  threads,  much  longer  than  cilia,  are  shown  in  the  lower  part  of 
Fig.  88,  B,  trch. 

Stained  preparations  should  be  examined  for  the  nucleus.  A 
small  nucleus  (paranucleus)  lies  near  the  main  nucleus. 

Division.  —  Some  of  the  paramecia  seen  swimming  may  be 
constricted  near  the  middle  of  their  bodies,  as  if  an  invisible 
thread  were  tied  around  them.  This  indicates  approaching 
division,  and  such  a  specimen  should  be  watched  as  the  con- 
striction grows  deeper,  and  finally  the  animal  is  completely 
divided  into  two  new  and  young  individuals  which  swim 
independently.  The  parent  animal  merges  its  own  individu- 
ality into  that  of  its  two  equal  offspring.  Contrast  this  with 
higher  forms,  as  the  frog,  which  produce  young,  while  the 
parent  continues  to  live  until  it  grows  old  and  dies.  It  is 
evident  that,  barring  accidents  and  disease,  there  is  no  chance 
fdr  a  paramecium  to  grow  old  and  die ;  for  when  it  grows 
old,  it  simply  divides  into  two  young  animals,  which  in  turn 
take  food,  form  new  protoplasm,  grow  to  the  full  size,  and 
divide.  This  is  the  characteristic  method  of  reproduction 


THE  SIMPLEST  ANIMALS 


303 


FIG.  89.     Division    or    re- 

SSf±tar°'fe 

Paramecium.    n,  nucleus; 

^™c}uole'    (From 


among  one-celled  animals.     It   is   simple   cell-division,  but 
the  two  new  cells  separate,  instead  of  remaining  together  as 
in  the  tissues  of  higher  plants  and  ani- 
mals.    This  is  a^^ua]^reprQ.dM(Man. 
Conjugation.  —  Under  certain  con- 
ditions, not  yet  entirely  understood, 
paramecia  reach  a  state  when  they  are 
unable  to  continue  to  divide.     Two 
such  individuals  come  into  contact, 
and  through  their  delicate  cell-walls 
some  of  the  nucleus  of  each  one  passes 
over  to  join  the  nucleus  of  the  other^. 
the  result  being  in  each  individual  a 

nfiBUlUcleUS,  half  of  which  has   come 

from     another     paramecium.     Then 

the    two    animals    SWim    away    inde- 

pendently,   each    soon   divides,    and 

their  offspring  may  continue  to  divide 

for  a  long  series  of  generations  before  this  exchange  of 
nuclear  substance  again  takes  place. 
This  process  is  known  as  conjugation 
and  in  essentials  is  similar  to  conjuga- 
tion in  molds  (§  244)  and  Spirogyra 
(§  239),  and  is  believed  to  have  the 
same  effect  on  heredity  as  has  fertili- 
zation in  higher  animals  (e.g.,  frog, 
§  58).  In  short,  the  conjugation  of  two 
paramecia  is  the  simplest  form  of  fer- 
tilization known  among  animals,  just 
as  that  of  Sphaerella,  molds,  and  Spi- 
rogyra is  the  simplest  in  plants.  Other 
kinds  of  one-celled  animals  are  known 
to  undergo  conjugation. 

268.  Physiology  of  Paramecium.  We 

are  now  ready  to  survey  the  life-activities  of  Paramecium. 


A  B 

FIG.  90.  Division  of  a 
Stentor  (trumpet-ani- 
malcule), n,  bead-like 
nucleus;  v,  vacuole. 
(From  Hatschek.) 


304  APPLIED  BIOLOGY 

Foods.  —  Like  every  other  animal,  it  requires  foods  which 
have  been  made,  directly  or  indirectly,  by  plants.  These 
foods  are  taken  in  through  its  mouth  and  gullet,  are  digested 
by  enzymes  which  are  believed  to  be  similar  to  those  which 
cause  digestion  in  higher  animals,  and  the  digested  foods 
are  absorbed  by  the  protoplasm.  Finally,  the  indigestible 
substances  are  ejected  from  the  body.  And  for  all  these 
processes  there  are  no  special  organs  like  stomach,  intestine, 
liver,  and  pancreas  in  a  frog. 

Oxygen.  —  Like  every  other  animal,  it  requires  oxygen.  It 
must  breathe;  but  it  has  no  lungs,  as  have  higher  animals. 
However,  its  delicate  cell-wall  allows  the  absorption  of  an 
abundance  of  oxygen  from  the  surrounding  water.  Para- 
mecia  soon  die  in  water  from  which  the  oxygen  has  been 
removed. 

Oxidation  and  Excretion.  —  Like  every  other  animal,  a  para- 
mecium's  food  is  constantly  being  oxidized  to  furnish  energy 
to  run  the  living  machine  (§  42),  and  this  union  of  oxygen 
and  foods  forms  excretions.  As  in  a  frog,  these  are  chiefly 
water,  carbon  dioxide,  and  nitrogenous  excretion.  The  car- 
bon dioxide  is  absorbed  by  the  surrounding  water.  The  water 
and  the  nitrogenous  excretion  is  pumped  out  by  the  regular 
pulsations  of  the  contractile  vacuoles.  Thus,  without  lungs, 
kidneys,  or  skin,  a  paramecium  gets  rid  of  the  same  excre- 
tions which  in  the  frog  and  other  higher  animals  must  be 
eliminated  by  these  organs. 

Assimilation. — As  in  every  other  animal,  some  food  must 
be  used  continually  for  making  new  protoplasm,  by  assimila- 
tion. Protoplasm  is  continually  wearing  out  and  some  food 
(especially  proteins)  must  be  used  for  making  new  particles 
of  protoplasm  to  replace  those  worn  out  or  destroyed.  Also, 
the  animals  are  frequently  dividing,  and  every  new  individual 
is  half  the  usual  size  and  must  make  enough  new  protoplasm 
to  double  its  size  before  it  can  reproduce.  Hence,  much  food 
must  be  used  to  make  new  protoplasm  for  growth  (§  42). 


THE  SIMPLEST  ANIMALS  305 

No  Circulation.  —  Especially  is  it  noteworthy  that,  unlike 
the  frog  and  other  higher  animals,  a  paramecium  carries 
on  its  life-activities  without  organs  for  circulation  (heart  and 
blood-vessels) .  Turn  back  and  review  the  reasons  why  a  frog 
needs  these  organs  (§  52),  and  it  will  be  evident  that  none  of 
the  reasons  given  applies  to  a  paramecium,  for  that  animal 
is  so  small  that  digested  food,  oxygen,  and  excretions  do  not 
require  transportation  to  and  from  distant  parts  of  the  body. 
It  is  only  a  short  distance  from  the  surface  of  the  body  to  the 
innermost  particles,  and  so  oxygen  and  carbon  dioxide  can 
diffuse  as  easily  as  they  go  through  the  wall  of  a  blood- 
capillary  in  a  frog.  Also,  the  movement  of  the  inner  layer 
of  protoplasm  in  a  paramecium  helps  distribute  food  while 
it  is  being  digested. 

Irritability.  —  A  paramecium  shows  no  nerves  or  other 
structures  which  appear  to  be  substitutes  for  the  nervous 
system  in  a  frog ;  nevertheless  the  animal  responds  to  stimuli 
caused  by  chemicals,  touch,  light,  and  electricity.  It  has 
no  nervous  organs,  but  it  has  irritability  (response  to  stimuli) 
in  a  simple  form. 

Motion. — A  paramecium  has  movement,  and  yet  there 
are  no  visible  muscles.  Instead,  the  protoplasm  of  its  entire 
body  seems  to  have  the  power  of  movement  or  contractility. 
This  is  especially  developed  in  the  protoplasm  composing  the 
cilia. 

Reproduction.  —  And  finally  a  paramecium  has  the  power 
of  reproducing,  without  which  its  species  would  soon  cease 
to  exist.  Since  it  consists  of  a  single  cell,  its  only  possible 
way  of  producing  new  individuals  is  by  cell-division  a  simple 
process  of  asexual  reproduction.  And  to  provide  for  fre- 
quent mixing  of  protoplasm  from  two  individuals,  which  is 
of  great  significance  in  heredity,  Paramecium  also  has  a 
simple  process  of  fertilization  or  conjugation. 

Summarizing,  we  find  within  a  single  cell  of  this  simple 
animal  all  the  life-processes  which  we  find  in  higher  animals; 


306  APPLIED  BIOLOGY 

namely,  movement,  digestion,  absorption,  respiration,  assimi- 
lation, oxidation,  excretion,  irritability  (simple  nervous 
activity),  and  reproduction.  All  these  life-processes  are 
within  one  cell  in  a  paramecium,  and  require  tens  of  thou- 
sands of  cells  in  an  elephant.  But  there  is  a  difference  in  that 
the  paramecium  cannot  perform  any  process  as  well  as  can 
a  higher  animal.  In  fact,  to  a  large  extent  a  paramecium  is 
doing  just  what  every  individual  cell  in  any  higher  animal 
is  continually  doing  in  carrying  on  its  own  life-processes. 

269.  Physiological  Division  of  Labor.  —  The  fact  that  the 
cells  in  higher  animals  are  specialized  to  do  certain  work 
(e.g.,  muscle  cells  to  contract  or  cause  movement,  stomach- 
cells  to  digest,  kidney  cells  to  excrete,  nerve  cells  for  co- 
ordination, etc.),  is  known  as  physiological  division  of  labor. 
The  advantage  of  such  specialization  is  shown  by  an  analogy 
from  human  society.  Each  pioneer  in  the  country  regions  in 
America  had  to  be  his  own  baker,  miller,  carpenter,  black- 
smith, cobbler,  etc.,  because  special  workers  in  these  lines 
were  not  near.  But  such  a  man  never  became  expert  in  any 
of  these  lines ;  he  was  "  a  Jack  of  all  trades,  master  of  none/' 
Nowadays  in  civilized  communities  a  physiological  division  of 
labor  has  led  men  to  become  specialists  and  learn  to  do  one 
thing  excellently.  But  there  must  be  a  coordination  or  a 
working  together.  One  man  may  specialize  as  a  carpenter ; 
but  he  must  depend  upon  other  men  to  be  his  cobbler,  grocer, 
baker,  farmer,  and  so  on  through  a  long  list  of  people  who 
must  do  things  for  the  one  who  confines  his  work  to  one  special 
line.  In  our  great  cities  we  do  not  often  stop  to  think  of 
this  physiological  division  of  labor  which  has  grown  up  in 
our  complex  human  society;  but  if  all  the  grocers  were  to 
close  their  shops,  or  as  has  actually  happened,  the  railroad 
engineers  should  stop  work  and  leave  us  without  supplies, 
then  we  could  realize  the  complex  way  in  which  we  have  be- 
come dependent  upon  other  workers  in  special  lines  of  work. 

All  this  from  human  society  illustrates  the  division  of  labor 


THE  SIMPLEST  ANIMALS  307 

in  higher  animals.  For  example,  muscle  cells  have  specialized 
for  movement ;  but  for  them  other  cells  must  do  the  digest- 
ing, absorb  the  oxygen,  discharge  the  excretions,  etc.  In 
short,  the  cells  of  every  organ  depend  upon  the  cells  of  all 
the  other  organs ;  and  all  must  work  together  harmoniously, 
because  there  is  mutual  interdependence.  And  just  as  a  man 
specializing  in  one  business  becomes  expert,  so  a  cell  specializ- 
ing in  movement,  secreting,  or  in  any  other  one  necessary 
function  can  do  that  work  better  than  it  is  done  in  a  parame- 
cium,  for  that  animal  has  so  many  things  to  do  in  its  one  cell 
that  it  does  none  of  them  as  well  as  they  are  done  by  the 
specialized  cells  in  higher  forms  of  life. 

270.  Amoeba.  —  In  many  ways  the  most  interesting  of 
one-celled  animals  is  one  known  to  biologists  as  amoeba, 
which  lives  on  the  surface  of  submerged  objects  in  ponds  and 
other  bodies  of  water. 

(D)  If  one  scrapes  the  ooze  from  the  lower  surface  of  lily  leaves, 
and  from  various  other  aquatic  leaves  and  stems,  or  collects  some  of 
the  green  felt-like  growth  which  often  forms  a  coating  on  the  mud 
in  the  bottom  of  shallow  stagnant  ponds  or  streams,  it  is  probable 
that  some  amoebas  will  be  obtained.  But  they  are  rarely  as  abun- 
dant as  paramecia,  and  so  it  is  usually  necessary  to  spread  the  collected 
material  in  soup-plates  or  other  shallow  dishes  and  allow  a  few  days 
in  which  the  amoebas  may  crawl  to  the  surface,  and  perhaps  also 
increase  in  numbers.  From  time  to  time  collect,  with  a  rubber- 
bulbed  pipette,  some  of  the  scum,  especially  near  the  edge  of  the 
dish,  spread  on  glass  slides,  and  with  a  low-power  objective  look 
for  little  specks  having  the  appearance  of  ground  glass,  and  irregu- 
lar in  form,  as  represented  in  Fig.  91.  Sometimes  it  is  necessary  to 
examine  dozens  of  slides  with  material  from  different  parts  of  the 
dish  before  an  amoeba  is  located.  It  is  only  in  this  way  that  amoebas 
can  be  found,  for  they  are  rarely  larger  than  T^  inch  in  diameter 
and  usually  very  much  smaller. 

Having  located  a  good  specimen,  look  at  the  cover-glass  from  the 
side  of  the  objective  and  estimate  the  approximate  position  of  the 
animal,  so  that  if  the  slide  is  moved,  it  will  be  easy  to  find  the 
place  again.  Then  carefully  adjust  a  higher  power  objective,  and 
study  according  to  the  following  description. 


308 


APPLIED  BIOLOGY 


The  body  of  an  amoeba  consists  of  protoplasm,  imbedded 
in  which  are  many  food  particles  that  have  been  eaten  and 
not  yet  digested.  The  outermost  protoplasm  is  perfectly 
transparent  and  colorless,  while  the  central  portion  is  very 
granular  and  resembles  ground  or  frosted  glass.  The  micro- 
scope and  drawings  give  the  impression  that  the  animal  is  a 
thin  sheet  of  protoplasm,  spread  out  like  a  liquid  poured  on 


FIG.  91.  Amoeba.  Arrows  show  direction  of  flowing  of  the  protoplasm 
n,  nucleus  ;  fv,  food  vacuole  ;  wv,  water  vacuole  ;  cv,  contractile  vacuole  ; 
p,  temporary  posterior  end.  (From  Wilson.) 

a  flat  plate ;  but  when  the  animal  is  disturbed  (as  by  jarring) 
it  rolls  up  into  a  rounded  mass. 

Movement.  —  The  most  noticeable  thing  about  an  active 
amoeba  is  that  it  is  continually  changing  its  shape.  Close 
observation  will  show  that  this  is  due  to  the  fact  that  the 
animal  is  composed  of  semi-fluid  protoplasm  which  is  con- 
stantly flowing.  This  movement  of  the  protoplasm  is  not 
like  the  flowing  of  water  down  an  incline,  but  is  due  to  con- 
tractions originating  within  the  amoeba  itself.  It  is  possible 
for  the  animal  to  crawl  up  the  stem  of  a  water  plant,  and 
this  could  not  be  explained  by  the  laws  of  gravitation. 


THE  SIMPLEST  ANIMALS  309 

When  the  animal  is  actively  moving,  long  arm-like  pro- 
jections, called  pseudopods  (false  feet),  are  formed,  and  then 
granules  from  the  main  body  continue  streaming  in  the 
same  direction.  In  fact,  the  motion  reminds  one  of  the  flow- 
ing of  lifeless  liquids,  and  we  can  imitate  it  by  pouring  some 
thick  liquid  (molasses,  melted  gelatin,  or  mucilage)  on  a  plate, 
and  then  by  tilting  cause  the  liquid  to  flow  in  various  direc- 
tions. Notice  that  when  a  streamlet  starts  in  one  direction, 
the  liquid  all  tends  to  flow  in  the  same  direction. 


FIG.  92.     Diagrams  1-6  show  how  an  amoeba,  or  a  similar  white  cell  from 
blood  of  higher  animals,  passes  through  very  narrow  openings. 

Sometimes  a  nucleus  can  be  seen  in  a  living  amoeba,  but 
it  is  most  evident  in  a  stained  preparation. 

In  large  specimens  it  is  easy  to  see  the  one  contractile  vacuole, 
which  acts  essentially  like  those  seen  in  a  paramecium,  and 
has  the  same  function  of  excreting  water  containing  nitroge- 
nous waste. 

Food.  —  The  taking  of  food  can  sometimes  be  observed 
when  an  amoeba  is  moving  actively.  If  it  comes  in  contact 
with  a  small  animal  or  plant,  two  .pseudopodia  flow  out  and 
gradually  surround  and  inclose  the  food  particle  in  the  pro- 
toplasm of  the  amoeba.  A  small  amount  of  water  surrounds 
the  food  particle,  just  as  in  a  paramecium.  Sometimes  an 
amceba  is  seen  so  filled  with  food  particles  that  the  body 
substance  is  opaque.  The  protoplasm  of  amcebas  secretes 
digestive  enzymes  which  dissolve  the  proteins,  etc.,  in  the 
animals  and  plants  which  are  captured.  The  digested  foods 
are  then  absorbed  by  the  surrounding  particles  of  protoplasm, 


310 


APPLIED  BIOLOGY 


FIG.  93.      Diagrams  showing  division  of  an  amoeba 
into  two.     n,  nucleus  ;  v,  vacuole. 


and  when  the  amoeba  moves  the  particles  being  digested  are 
circulated  so  that  dissolved  food  is  widely  distributed.  In- 
digestible particles  (hard  parts  of  small  animals  and  cell-walls 
of  plants)  are  from  time  to  time  ejected ;  the  protoplasm  of 
the  amoeba  appears  to  flow  away  from  them  as  water  on  a 
board  might  for  a  time  surround  and  inclose  some  grains  of 
sand  and  then  flow  away  and  leave  them  behind. 

Respiration.  —  Amoeba  breathes  like  a  paramecium;  that 

is,  simply  ab- 
sorbs oxygen 
from  the  sur- 
rounding water. 
Excretion  is 
the  same  as  in 
a  paramecium; 
carbon  dioxide 
is  absorbed  by 
the  surrounding 

water,  and  water  and  nitrogenous  excretions  are  pumped 
out  by  the  contractile  vacuole. 

Reproduction. —  Amcebas  are  rarely  seen  dividing,  but  if 
kept  in  watch-crystals  they  multiply  rapidly,  making  it 
evident  that  they  must  undergo  division  frequently.  Figure 
93  shows  the  process  of  division.  Conjugation  of  two  amoebas 
similar  to  that  of  the  paramecium  (§  267)  has  been  seen. 
Under  conditions  unfavorable  to  the  usual  activity,  amoebas 
will  sometimes  become  rounded,  and  secrete  around  them- 
selves a  hard  wall  or  cyst.  They  are  said  to  be  encysted.  In 
this  condition  they  are  able  to  live  for  some  time  without 
food  and  in  a  dry  state.  Such  a  habit  insures  their  existence 
in  dry  seasons  when  the  ponds  are  dry.  Also,  in  the  dry 
condition  they  may  be  widely  scattered  by  winds. 

271.  Simple  Life  of  Amoeba.  —  Compared  with  the  para- 
mecium, the  amoeba  is  exceedingly  simple  in  structure  and 
functions.  The  paramecium  has  a  constant  and  definite 


THE  SIMPLEST  ANIMALS  311 

form,  a  firm  but  delicate  cuticle  over  the  whole  surface, 
special  organs  of  locomotion  (the  cilia),  special  organs  for 
defense  (the  trichocysts),  a  definite  organ  for  receiving  food 
(groove  and  gullet),  and  differentiation  into  anterior  and 
posterior  ends.  Since  none  of  these  is  found  in  amceba, 
paramecium  is  obviously  a  much  higher  type  of  animal. 
The  amceba  is  the  simplest  animal  known,  and  we  cannot 
imagine  how  it  could  be  made  simpler  except  by  the  absence 
of  a  contractile  vacuole.  Such  an  amceba  has  been  described, 
in  which  apparently  the  surrounding  water  absorbs  such 
excretions  as  the  vacuoles  of  other  amcebas  pump  out. 

Amceba  appears  to  have  no  cell-wall  or  bounding  mem- 
brane. The  line  visible  where  the  water  touches  it  is  simply  a 
line  of  separation  between  water  and  the  protoplasm  of  the 
animal.  If  a  drop  of  oil  is  put  into  water,  there  appears  to 
be  a  line  where  the  oil  and  water  meet ;  but  there  is  no  special 
membrane  there,  simply  a  surface  film  between  the  oil  and 
water.  Likewise,  we  see  insects  striding  on  the  surface  of 
water,  and  it.  is  easy  to  make  a  fine  sewing  needle  float  on  the 
surface  of  water ;  but  no  one  thinks  of  the  water  as  covered 
with  a  specially  differentiated  membrane. 

In  short,  an  amceba  is  a  mass  of  simple  protoplasm,  very 
similar  to  the  inside  material  in  a  paramecium,  and  to  that 
inside  the  cells  of  higher  animals.  The  animal  has  long  been 
a  favorite  with  biologists  because  it  shows  the  appearance 
of  protoplasm,  which  cannot  be  seen  living  in  the  opaque 
cells  of  higher  animals. 

272.  Economic  Relations  of  Amoebas.  —  There  are  many 
species  of  amcebas,  and  most  of  them  are  not  known  to  be  of 
any  special  economic  interest;  but  one  species  common  in 
tropical  countries  produces  a  serious  form  of  intestinal  disease 
known  as  dysentery. 

273.  Allies   of   Amoeba   and    Paramecium :    Protozoa.  — 
These  animals  are  members  of  the  lowest  division  of  the 
animal  kingdom,  a  group  of  animals  which  are  characterized 


312  APPLIED  BIOLOGY 

by  the  fact  that  their  bodies  consist  of  one  cell.  Any  one- 
celled  animal  is  a  protozoan  (meaning  first  animal),  and  the 
entire  group  of  them  is  named  Protozoa.  There  are  numerous 
species,  but  most  of  those  known  are  like  the  amoeba  and 
paramecium  in  being  of  little  interest  except  that  study 
of  them  has  helped  much  towards  explaining  the  life  of  higher 
organisms,  including  man  himself.  However,  a  few  proto- 
zoans are  now  known  to  be  of  very  great  practical  interest, 
and  some  of  these  will  be  briefly  described. 

274.  Malarial  Organism.  —  Malaria  in  its  severest  forms 
has  long  been  one  of  the  worst  diseases  affecting  the  human 
race.  Vast  territories  in  some  parts  of  the  world  have  been 
left  practically  undeveloped  by  civilized  men  because  of 
malaria.  Little  was  known  as  to  the  cause  of  the  disease 
until  after  1880,  in  which  year  a  protozoan  parasite  was 
discovered  in  red  blood-cells.  Before  that  time  it  was  com- 
monly supposed  to  be  caused  by  some  poisonous  vapor  or 
miasm  which  arose  from  swampy  land.  In  1897,  Ross,  an 
officer  of  the  British  army,  demonstrated  that  the  malarial 
parasites  first  develop  in  the  stomachs  of  certain  mosquitoes 
(§  329),  from  blood  sucked  from  persons  who  have  malaria; 
and  a  year  later  other  investigators  showed  that  mosquitoes 
which  have  obtained  blood  from  a  malarious  patient  are  able 
to  transmit  the  parasites  while  sucking  blood  from  perfectly 
healthy  persons.  Many  later  studies  have  made  it  absolutely 
certain  that  the  Anopheles  mosquito  is  the  carrier  of  the  dis- 
ease ;  and  this  is  one  reason  for  the  recent  attempts  at  ex- 
terminating mosquitoes  as  far  as  possible. 

The  effect  of  the  malarial  parasite  upon  the  red  blood- 
corpuscles  is  as  follows  :  Small  bodies  injected  into  the  blood 
by  sucking  mosquitoes  attach  themselves  to  blood-cells  and 
begin  to  burrow  (they  have  amoeboid  (amoeba-like)  move- 
ments). Within  the  blood-cell  a  parasite  grows,  at  the  ex- 
pense of  the  cell,  and  soon  divides  into  a  number  (6-24)  of 
small  bodies  called  spores.  Then  the  blood-cell  disintegrates 


THE  SIMPLEST  ANIMALS  313 

and  frees  the  spores,  which  fasten  themselves  to  new  blood- 
cells  and  repeat  the  development.  Each  time  the  spores  are 
freed  by  breaking  of  the  blood-cells  the  patient  has  the  chill 
and  fever  which  are  characteristic  of  the  disease.  It  is  well 
known  that  malarial  attacks  commonly  occur  on  alternate 
days,  that  is,  every  forty-eight  hours.  This  means  that  a 
common  form  of  malarial  parasite  requires  two  days  for  de- 
velopment in  a  blood-cell.  Another  variety  of  malarial  para- 
site takes  three  days  for  development  into  spores,  and  so  the 
chill  of  the  patient  comes  at  intervals  of  three  days.  Still 
another  form  of  malaria,  most  common  in  the  tropics,  and 
exceedingly  difficult  to  cure,  may  appear  daily;  and  the 
parasites  take  twenty-four  hours  for  their  development  in 
blood-cells. 

All  these  oft-repeated  cycles  of  development  in  human 
blood-cells  are  simply  growth  and  cell-division,  typical 
asexual  reproduction.  When  human  blood  is  sucked  into 
the  stomach  of  a  mosquito,  pairs  of  certain  spores  unite  (a 
true  case  of  fertilization)  and  the  zygote  (combined  cell) 
penetrates  the  stomach-wall  of  the  mosquito  and  becomes 
encysted.  Sometimes  there  are  several  hundred  such  cysts 
in  the  stomach  of  a  single  mosquito.  Each  of  these  encysted 
parasites  eventually  divides  into  thousands  of  slender  thread- 
like bodies  (sporozoites),  which  by  way  of  the  lymph-tubes 
get  scattered  in  all  parts  of  the  mosquito's  body,  especially 
in  the  poison  (salivary)  glands.  It  is  these  sporozoites  which 
are  carried  along  with  the  poison  when  a  mosquito  plunges 
its  beak  into  human  blood-vessels;  and  each  sporozoite 
enters  a  red  blood-cell,  where  it  divides  into  6  to  24  spores. 

The  full  development  in  a  mosquito's  body  need  not  take 
over  ten  days  in  warm  weather. 

When  once  started  in  human  blood  the  repeated  cycles 
may  go  on  for  a  long  time,  or  spores  remain  which  may  long 
afterwards  start  a  new  attack  of  disease.  This  can  be  pre- 
vented by  the  use  of  quinine,  which  kills  the  parasites,  especially 


314  APPLIED  BIOLOGY 

if  taken  in  large  doses  just  as  the  fever  is  subsiding,  which  is 
while  the  young  spores  are  free  in  the  blood-plasma.  Killing 
the  parasites  in  human  blood  prevents  infection  of  mosquitoes; 
and  if  all  people  could  be  treated  with  quinine  systematically, 
it  is  probable  that  the  disease  could  be  stamped  out.  This 
has  been  tried  under  the  direction  of  Dr.  Koch  with  great 
success  in  some  small  islands.  Preventing  the  spread  of  the 
malarial  parasite  by  preventing  the  breeding  of  mosquitoes 
and  by  protection  against  bites  will  be  discussed  under  insects 
and  disease  (§  329).  In  the  ten  years  since  it  was  found  that 
mosquitoes  are  responsible  for  the  spread  of  malaria,  a  great 
effort  has  been  made  to  exterminate  the  insects  in  many 
localities,  with  the  result  that  there  has  been  a  decided  decrease 
of  malaria  wherever  the  mosquitoes  have  become  rare.  Pro- 
tection against  bites  by  sleeping  in  screened  houses  and  by 
wearing  mosquito-proof  clothing  and  a  veil  when  outdoors 
at  night  has  greatly  reduced  the  number  of  cases  of  malaria 
among  the  employees  of  certain  railroad  systems  along  the 
Mediterranean  shore. 

275.  The  Parasite  of  Sleeping  Sickness.  —  The  terrible 
South  African  disease  known  as  the  sleeping  sickness,  from 
which  more  than  half  a  million  natives  in  the  Congo  region 
have  perished  in  the  last  ten  years,  is  now  known  to  be  due  to 
a  parasitic  protozoan,  which  is  injected  into  human  blood- 
plasma  by  the  bite  of  a  peculiar  African  fly.  The  parasite 
swims  freely  in  blood-plasma,  its  movement  being  due  to  long 
whip-like  structures,  called  flagella.  It  belongs  to  a  group 
of  Protozoa  known  as  the  trypanosomes.  Many  experts  on 
parasitic  diseases  are  now  engaged  in  studying  the  sleeping 
sickness;  but  so  far  no  satisfactory  remedy  has  been  dis- 
covered. It  is  spreading  rapidly,  and  has  become  one  of  the 
greatest  problems  in  Africa. 

A  similar  parasite,  injected  by  a  similar  fly,  causes  the 
disease  of  horses,  mules,  and  oxen  called  "nagana"  by  the 
natives,  and  by  them  correctly  charged  to  the  tsetse-fly. 


THE  SIMPLEST  ANIMALS  315 

Those  who  have  read  the  "  Travels  "  of  Livingstone  and  other 
African  explorers  will  recall  how  their  journeys  were  so  often 
delayed  by  death  of  their  oxen.  The  disease  also  caused  much 
trouble  among  horses  during  the  South  African  war  a  few  years 
ago.  The  fly  does  not  develop  the  parasite,  as  mosquitoes 
do  in  the  case  of  the  malarial  organism,  but  simply  transmits 
the  parasites  from  diseased  to  healthy  animals.  It  is  believed 
that  the  blood  of  some  large  game  animals  may  infect  the  flies. 

The  surra  disease,  which  destroys  large  numbers  of  horses, 
camels,  and  cattle  in  India  and  the  Philippines,  is  due  to  a 
similar  parasite  transmitted  by  bites  of  flies. 

276.  Other  Protozoan  Diseases.  —  Dangerous  diseases, 
caused  by  protozoan  parasites  which  are  introduced  into  the 
blood  by  the  bites  of  ticks  (§  319),  affect  cattle,  dogs,  horses, 
and  sheep.  The  parasites  enter  the  red  blood-cells.  Texas 
fever  is  one  form  of  the  cattle  disease,  and  it  has  caused 
enormous  financial  losses  to  stockmen  in  the  United  States 
and  elsewhere. 

A  common  disease  of  the  liver  in  rabbits,  diseases  which 
seriously  affect  fish,  the  silkworm  disease  made  famous  by 
Pasteur's  studies,  —  these  are  some  of  the  more  important 
cases  in  which  parasitic  one-celled  animals  have  great  eco- 
nomic importance. 

There  are  some  suggestions  that  yellow  fever,  scarlet  fever, 
smallpox,  and  hydrophobia  are  caused  by  protozoan  parasites 
similar  to  those  which  cause  malaria ;  but  there  is  yet  no  con- 
vincing proof.  In  the  case  of  yellow  fever,  it  has  been  proved 
that  a  mosquito  (not  the  species  concerned  with  malaria) 
transmits  the  unknown  germ  of  the  disease ;  and  so  at  pres- 
ent the  only  known  way  of  checking  the  spread  of  yellow 
fever  is  to  check  the  mosquitoes.  This  has  been  very  suc- 
cessful in  Cuba  and  in  Panama. 

Fortunately,  most  of  the  protozoans  which  live  in  water 
in  lakes,  rivers,  and  sea  are  unable  to  live  in  the  bodies  of 
animals,  and  hence  are  entirely  harmless. 


316  APPLIED  BIOLOGY 

277.  Useful     Protozoa.  —  The    emphasis    upon     certain 
protozoans  as  causes  of  disease  tends  to  leave  the  false  im- 
pression that  all  protozoa  are  harmful,  or  at  least  useless. 

A  large  number  of  common  protozoans  are  important  in  the 
food-supply  of  somewhat  larger  animals,  these  in  turn  of  still 
larger  animals,  and  so  on  to  aquatic  animals  such  as  fishes. 

Thus  even  the  minute  bacteria  eaten  by  some  protozoans 
may  indirectly,  through  a  series  of  animals  of  increasing  size, 
finally  come  to  be  of  use  in  the  food-supply  of  man  himself. 
We  cannot  accurately  estimate  the  relation  of  the  smallest 
animals  in  the  food-supply  of  larger  animals ;  but  vast  num- 
bers of  animals  certainly  depend  directly  and  indirectly  upon 
protozoans  as  food. 

Many  of  the  protozoans  are  important  as  scavengers,  as- 
sisting bacteria  in  breaking  up  dead  organic  bodies  (§§  116, 
117)  and  preparing  the  organic  material  for  use  again  in  the 
cycle  of  organic  matter.  For  example,  a  protozoan  may  eat 
some  particles  from  the  body  of  a  dead  animal  or  plant,  build 
these  particles  into  its  own  protoplasm,  which  may  later  serve 
as  protein  food  for  some  larger  animal ;  or  these  particles 
may  soon  be  oxidized  to  excretions  which  may  serve  as  ma- 
terials for  food  of  plants. 

Another  example  of  the  usefulness  of  protozoans  is  that  a 
large  amount  of  chalk  in  the  great  deposits  in  England  and 
elsewhere  is  composed  of  the  shells  of  certain  protozoans. 
In  some  places  these  shells  are  very  abundant  in  the  mud  of 
the  ocean  bottom. 

278.  Colonial  Protozoa.  —  Amceba  and  Paramecium  rep- 
resent the  common  kinds  of  protozoans  in  that  all  individuals 
live  free  and  independently  of  each  other.     We  shall  examine 
briefly  groups  or  colonies  of  certain  protozoans. 

One  of  the  most  common  colonial  protozoans  is  a  tree-like 
form.  It  consists  of  a  delicate  and  much-branched  stalk, 
which  is  attached  to  some  object,  and  at  the  end  of  each 
branch  is  a  one-celled  animal  of  bell-like  form  (similar  to  Fig. 


THE  SIMPLEST  ANIMALS 


317 


b 


94,  6) .  Some  of  these  individuals  can  take  food  (by  the  action 
of  cilia  which  drive  food  particles  into  the  mouth,  as  in  a 
paramecium),  and  can  divide  to  form  new  individuals  on 
new  branches.  Some  of  the  individuals  do  not  feed  while 
attached  to  the  stalk  of 
the  colony,  but  digested 
food  comes  to  them 
through  the  hollow  stalk 
from  others  which  do  take 
and  digest  food.  These 
specialized  non-feed- 
ing individuals  break 
loose,  swim  freely  for  a 
time,  attach  themselves 
to  some  object,  develop 
a  mouth,  and  soon  divide 
to  form  two  new  individ- 
uals. Then  division  is 
repeated  again  and  again 
and  finally  results  in  a 
new  tree-like  colony. 

Note  that  in  such  a 
colony  there  is  a  slight 
division  of  labor  in  that 


FIG.  94.  Vorticella,  the  bell-animalcule,  a, 
group  of  seven  voiticellas  attached  to  the 
surface  cells  of  a  water-plant.  The  con- 
tractile stalk  is  thrown  into  a  spiral  when 
shortened  (see  a  and  c).  6,  a  single  in- 
dividual ;  d,  e,  division  ;  /,  a  free-swim- 
ming individual  formed  by  division  as  in 
d,  e.  (From  Wilson,  and  Parker.) 


some  of  the  individuals 
are  nutritive,  i.e.,  adapted 
to  feeding  and  digesting 
for  the  benefit  of  the  whole  colony ;    and  others  are  special- 
ized for  reproducing  new  colonies. 

Vorticella.  —  These  are  similar  protozoans,  the  individuals 
of  which  live  separately.  An  example  is  the  beautiful  bell- 
animalcule  (Vorticella),  several  of  which  are  shown  in  Fig. 
94,  a.  The  food-collecting  groove  is  at  the  wide  end  of  the 
bell-shaped  body,  and  a  circle  of  cilia  whips  particles  around 
the  margin  and  down  into  a  gullet.  The  stalk  contains  a  cen- 


318 


APPLIED  BIOLOGY 


tral,  highly  contractile  fiber  (the  simplest  muscle  known), 
which  frequently  shortens  (contracts)  and  coils  the  stalk  into 
a  close  spiral  (Fig.  94,  c).  The  slightest  jar  of  the  micro- 
scope, the  touch  of  another  organism,  or  even  a  current  of 
water  will  cause  sudden  contraction. 

Vorticella  multiplies  by  division  (Fig.  94,  d,  e).     One  of 
the  two  individuals  thus  formed  leaves  the  stalk  and  becomes 

free  swimming  (Fig.  94,  /). 
Soon  it  settles  down,  grows 
a  stalk  of  its  own,  and  be- 
comes a  full-grown  vorticella, 
ready  to  reproduce  by  di- 
vision. 

If  we  imagine  that  the  vor- 
ticellas  formed  by  division 
remain  on  branches  of  the 
same  stalk,  then  division  re- 
peated many  times  would 
produce  a  tree-like  colony 
with  individual  animals  in 
the  same  positions  as  are  the 
terminal  buds  on  branches  of 
plants. 

279.  Volvox:  Colonial  Ani- 
mal or  Plant  ? — The  most  re- 
markable of  colonial  organisms  is  the  beautiful  Volvox  (Fig. 
95),  which  lives  in  ponds  of  fresh  water.  The  colonies, 
which  are  visible  to  the  unaided  eye,  are  hollow  spheres  of 
a  transparent  gelatinous  material  in  which  are  set  numerous 
individuals  (single  cells),  each  with  two  flagella.  Each  in- 
dividual cell  has  a  nucleus,  a  chlorophyll-body,  and  a  red 
spot.  The  combined  lashing  of  all  the  flagella  causes  the 
colony  to  roll  through  the  water;  hence  the  name  Volvox 
was  made  from  a  Latin  word  meaning  to  roll. 
The  eight  dark  spheres  represented  in  the  figure  are  new 


FIG.  95.  Colony  of  Volvox.  Each 
zooid  has  two  cilia.  Eight  young 
colonies  are  shown  in  the  interior 
of  the  hollow  sphere,  a,  colonies 
just  beginning  to  form  from  certain 
zooids  at  the  surface.  (From 
Parker.) 


SIMPLEST  ANIMAL8  319 

colonies  forming  inside  the  old  colony.  Each  of  the  new 
colonies  is  started  by  one  cell  (individual)  moving  inward 
and  dividing  up  into  a  large  number  of  cells,  which  form  a 
new  colony.  Finally,  the  outer  sphere  breaks  and  releases 
the  new  colonies. 

Sometimes  Volvox  has  sexual  reproduction.  Certain  cells 
enlarge  and  become  egg-cells.  Some  other  cells  form  sperm- 
cells.  Each  fertilized  egg-cell  divides  into  a  large  number 
of  cells  which  form  a  new  colony. 

Biologists  are  disagreed  as  to  whether  Volvox  is  an  animal 
or  a  plant.  It  is  commonly  described  in  textbooks  of  both 
botany  and  zoology.  The  truth  is  that  it  is  one  of  many 
small  organisms  —  some  living  singly  and  some  in  colonies  — 
which  so  combine  characters  of  both  animals  and  plants  that 
they  may  be  said  to  be  near  or  on  the  boundary  line  between 
the  animal  and  plant  kingdoms.  Volvox  behaves  like  an 
animal,  but  its  nutrition  is  that  of  a  green  plant.  In  fact, 
each  cell  of  a  Volvox  colony  is  comparable  to  a  single  Sphae- 
rella  (§  238) .  It  is  probably  a  colony  of  one-celled  plants,  one 
of  the  Algae. 

However,  whether  Volvox  is  animal  or  plant  is  a  point  of 
little  importance,  for  it  is  an  organism  illustrating  colonial  life 
of  one-celled  living  things  so  well  that  it  deserves  to  be  studied 
in  connection  with  both  botany  and  zoology. 

Volvox,  like  the  colonial  bell-animalcules,  has  some  cells 
for  nutrition,  and  some  specialized  for  reproduction.  In  the 
next  animals  to  be  studied  we  shall  find  this  division  of  labor 
more  fully  developed. 

Classes  of  Protozoa  (One-Celled  Animals) 

Sarcodina  —  simple  masses  of  protoplasm,  with  pseudopodia. 
Amreba. 

Sporozoa  —  spore-animals  ;    producing  spores.     Malaria  germs. 

Mastigophora  ; —  with   flagella   for   swimming.     Volvox. 

Infusoria  —  with  cilia  for  swimming  and  collecting  food.  Para- 
mecium,  Vorticella. 


CHAPTER  XI 

THE  SIMPLEST  MANY-CELLED  ANIMALS  :  PORIFERA 
AND  CCELENTERATA 

280.  Many-Celled  Animals  :  Metazoa.  — All  animals  higher 
than  the  Protozoa  consist  of  many  cells.     There  is  no  animal 
with  simply  two  cells   or   four  cells  in  the  adult  condition, 
although  all  many-celled  animals  in  developing   from  eggs 
must  pass  through  two-cell  and  four-cell  stages,  as  does  the 
frog  (§  59). 

All  the  many-celled  animals  taken  together  are  termed 
Metazoa.  This  is  simply  a  convenient  short  term  in  place  of 
the  phrase  "  animals  with  many  cells."  Familiar  repre- 
sentatives of  the  various  types  of  metazoa  are  sponge-animals, 
coral-animals  and  jelly-fishes,  worms,  lobster  and  insects, 
clam  and  snail,  starfish,  and  frog.  These  are  some  members 
of  the  larger  divisions  (phyla)  of  the  animal  kingdom.  See 
table  in  §  133. 

All  the  many-celled  animals  exhibit  more  or  less  physiologi- 
cal division  of  labor  (§  269) .  In  some  of  those  which  we  shall 
study  first  there  are  some  groups  of  cells  assigned  to  certain 
functions,  but  no  such  separation  into  special  organs  as  we 
found  in  the  frog.  The  most  distinguishing  feature  of  the 
life-activities  of  the  animals  described  in  this  chapter  is  that, 
although  each  one  of  them  may  have  hundreds  or  thousands 
of  cells  in  its  body,  they  are  able  to  live  without  blood  or 
similar  circulating  medium.  We  shall  see  that  this  is  due 
to  their  simple  plan  of  structure. 

PORIFERA 

281.  Sponge-animals.  —  Simplest  of  the  Metazoa  are  the 
animals  which  produce  the  fibrous  structures  known  as  sponges. 

320 


THE  SIMPLEST  MANY-CELLED  ANIMALS 


321 


These  are  the  skeletons  of  colonies  of  animals,  the  flesh  hav- 
ing been  removed  when  preparing  the  sponges  for  market. 
Besides  these  common  sponges,  there  are  certain  sponge- 
animals  which  have  skeletons  made  up  of  glass  spicules,  and 
others  which  form 
their  skeletons  of  lime 
(calcareous)  spicules. 
These  various  mate- 
rials found  in  sponge- 
animals  are  secreted 
by  living  cells,  just 
as  the  cells  of  higher 
animals  secrete  bone. 
The  general  plan  of 
one  of  the  simplest 
sponge-animals  is 
shown  in  Fig.  96.  On 
the  outside  are  nu- 
merous pores.  There 
is  a  central  cavity, 
which  has  an  opening 
at  the  top.  This  cen- 
tral cavity  is  con- 
nected by  small  tubes 
(see  the  figure)  with 
the  pores  on  the  sur- 
face of  the  body,  and 
water  flows  in  through 
these  tubes  or  canals 
and  out  at  the  top  of 
the  central  cavity.  As  shown  in  the  figure,  the  canals  starting 
from  the  external  pores  do  not  run  quite  to  the  central  cavity, 
and  other  canals  starting  from  the  central  cavity  extend  al- 
most to  the  surface.  Between  these  inner  and  outer  canals 
are  small  openings;  so  that  water  entering  the  outer  canal  can 

Y 


FIG.  96.  A  simple  sponge,  the  branch  at  the 
right  cut  to  show  central  cavity,  o,  opening 
of  central  cavity;  ip,  inhalent  pores.  The 
young  branch  at  left  has  been  formed  by 
budding.  (From  Parker  and  HaswelL) 


322  APPLIED  BIOLOGY 

pass  through  to  the  inner  canal  and  thence  into  the  central 
cavity.  The  inner  canal  is  lined  with  numerous  cells,  each 
of  which  has  a  long,  whip-like  structure  (flagellum) ;  and  it  is 
the  lashing  of  all  the  flagella  which  causes  water  to  flow  in 
through  the  pores. 

Structure  of  a  Simple  Sponge.  —  (D)  Grantia  is  the  name  of  a  small 
marine  sponge  found  attached  to  stones,  piles  of  wharves,  etc.  Schools 
far  from  the  sea  must  use  specimens  preserved  in  alcohol.  While 
observing,  keep  the  specimens  covered  with  alcohol  or  water  in 
watch-glasses,  or  in  small  bottles,  one  specimen  in  each  bottle.  Have 
some  specimens  entire,  and  some  split  longitudinally.  Use  a  hand- 
lens,  and  identify  all  structures,  except  cells,  mentioned  above. 
Some  cross  sections,  cut  with  a  razor,  should  be  demonstrated  to  show 
the  spicules  which  form  the  skeleton.  That  these  are  calcareous 
spicules  can  be  shown  by  pouring  some  acid  on  them.  The  fleshy 
substance  (i.e.,  the  cells)  can  be  cleaned  away  by  boiling  a  Grantia 
sponge  in  a  strong  solution  of  caustic  potash. 

The  feeding  of  a  sponge-animal  is  similar  to  that  of  a  para- 
mecium.  The  cells  with  flagella,  in  the  inner  canals,  are 
able  to  take  in  and  digest  food  particles  brought  by  the  in- 
coming currents  of  water.  Indigestible  particles  are  carried 
out  of  the  central  cavity  by  currents  of  water.  Those  cells 
which  are  not  fitted  to  take  in  and  digest  food  may  obtain 
digested  food  by  osmosis  from  the  other  cells.  Here  is  a 
simple  step  in  physiological  division  of  labor;  the  outside 
cells  are  the  protective  covering  of  the  body,  and  some  of  the 
inner  cells  digest  food. 

The  outer  cells  are  collectively  known  as  the  ectoderm 
(meaning  outer  skin),  while  the  inner  layer  of  cells  which 
line  the  inner  canals  and  central  cavity  and  do  the  digesting 
constitute  the  endoderm  (meaning  inner  skin). 

Reproduction.  —  Sponges  reproduce  by  outgrowths  or  buds 
from  the  older  animals,  and  when  the  animals  thus  formed 
remain  together  colonies  are  formed.  Figure  96  shows 
a  colony  of  three  individuals  formed  by  two  buds  on  the 


THE  SIMPLEST  MANY-CELLED  ANIMALS          323 

largest  one.  Also,  some  sponges,  especially  the  green  ones 
which  live  in  fresh  water,  form  structures  (called  gemmules) 
composed  of  cells  well  protected  by  a  hard  covering.  These 
gemmules  may  remain  inactive  until  the  following  summer, 
when  they  grow  into  new  individuals.  This  kind  of  repro- 
duction of  sponges,  like  the  budding,  is  asexual;  but  all 
sponge-animals  also  have  sexual  reproduction.  In  each 
animal  both  ova  and  sperm-cells  are  formed  from  certain 
inner  cells.  The  ova  are  usually  fertilized  by  sperm-cells 
produced  and  discharged  into  the  water  by  other  individuals 
and  carried  into  the  canals  by  incoming  currents  of  water. 
The  fertilized  eggs  develop  into  rounded  embryos  covered 
with  cilia  and  able  to  swim  freely.  After  a  time  these  settle 
down  on  rocks,  attach  themselves  by  secreting  a  cement-like 
substance,  and  grow  into  sponge-animals.  Each  animal 
developed  from  an  egg  may  later  by  branching,  as  described 
above,  form  a  large  colony  of  sponges. 

The  sponges  of  commerce  (horny  sponges)  have  the  same 
general  plan  of  structure  and  life  as  described  above  for  the 
simplest  sponges.  The  chief  difference  is  that  they  branch 
extensively  and  form  immense  colonies  whose  canals  and  cen- 
tral cavities  are  united  in  a  complicated  network.  An  ordi- 
nary large  sponge  is  the  skeleton  of  a  colony  of  many  sponge- 
animals.  Most  of  the  large  openings  seen  on  such  a  sponge 
are  the  united  central  cavities  from  a  number  of  animals  of 
the  colony,  and  the  smaller  openings  are  the  united  canals 
for  the  ingoing  of  water  to  many  individuals.  Each  individual 
animal  in  such  a  complicated  colony  has  a  structure  similar 
to  a  simple  sponge-animal. 

In  preparing  sponge  skeletons  for  market,  the  flesh  of  the 
masses  (colonies)  is  allowed  to  depay  by  the  action  of  bacteria, 
which  have  no  effect  upon  the  skeletons.  These  are  after- 
wards cleaned  by  chemical  solutions,  which  also  sterilize  them. 

Porifera.  —  Because  of  the  prominence  of  the  pores  for  the 
reception  and  expulsion  of  water,  the  sponge-animals  are 


324 


APPLIED  BIOLOGY 


known  as  the  Porifera  (meaning  pore-bearers).  This  is  the 
lowest  group  of  the  many-celled  animals.  It  is  a  noteworthy 
fact  that  the  Porifera  show  much  similarity  to  the  colonial  pro- 
tozoa, especially  in  that  food  is  digested  inside  of  cells,  and  not 
in  a  stomach  bounded  by  cells,  as  is  true  in  higher  animals. 

CCELENTERATA 

282.  Hydra.  —  Somewhat  more  differentiated  than  the 
sponge-animals,  but  still  exceedingly  simple  as  compared 
with  a  backboned  animal,  are  those  of  the  group  to  which 

belong  the  jelly-fishes  and 
coral-animals.  Most  of  these 
live  in  the  sea,  but  among  the 
few  species  which  live  in  fresh 
water  is  the  little  animal 
known  to  naturalists  as  Hy- 
dra, and  in  old  books  often 
called  "  fresh-water  polyps." 
It  commonly  lives  in  ponds 
and  streams,  where  it  clings  to 
aquatic  plants,  dead  leaves, 
and  sticks.  Such  objects 
should  be  collected,  placed  in 
glass  jars  with  water,  and  al- 
lowed to  stand  for  some  days. 
Many  of  the  hydras  will  move 
to  the  walls  of  the  glass  jar. 


FIG.  97.  Two  specimens  of  hy- 
dra, one  contracted  and  one 
expanded.  The  latter  has  three 
buds  in  stages  of  development. 
t,  tentacles;  a,  captured  water- 
flea;  hyp,  position  of  mouth. 
(From  Parker.) 


I.  (L)  Observe  hydras  in  glass 
jars  (aquaria)  near  windows.  In 
what  part  of  the  aquarium  with 
reference  to  light  and  shade  are 
they  most  abundant?  Note  the 
long  thread-like  arms  which  are 
attached  at  the  free  end  of  the 
animal.  Study  a  hydra  which  has 


THE  SIMPLEST  MANY-CELLED  ANIMALS          325 

been  removed  from  an  aquarium  to  some  water  in  a  watch-glass. 
Use  a  hand-lens.  What  is  the  shape  of  the  body  ?  Does  the  shape 
change?  Disturb  the  animal  by  touching  it  with  a  needle.  De- 
scribe what  happens.  How  may  this  reaction  be  of  use  to  the 
hydra  ?  Notice  that  one  end  becomes  attached  to  the  glass ;  this 
is  the  base.  Find  out  if  the  hydra  has  a  firm  hold  on  the  glass  dish, 
e.g.,  try  to  wash  it  off  with  a  gentle  jet  of  water  from  a  pipette. 
At  the  free  end  is  a  circle  of  tentacles.  How  many  ?  These  are  used 
for  catching  the  prey.  On  the  summit  of  a  conical  elevation  in  the 
center  of  this  circle  of  tentacles  is  a  small  opening,  the  mouth,  which 
is  usually  very  difficult  to  see.  How  do  the  tentacles  behave  when 
the  hydra  is  disturbed  ?  Can  the  tentacles  move  independently  of 
each  other?  Is  this  animal  bilaterally  symmetrical? 

Often  young  hydras  may  be  seen  attached  to  the  bodies  of  the 
larger  ones,  and  these  are  formed  by  outgrowths  or  buds  (Fig.  97). 

Make  an  outline  drawing  of  a  living  hydra,  showing  as  many 
structures  mentioned  above  as  possible — (1)  fully  extended, 
(2)  contracted. 

II.  (L)    Transfer  a  hydra,  with  a  little  water,  by  means  of  a 
clean  pipette,  to  an  object-slide.     Be  careful  to  support  the  cover- 
glass  (small  bits  of  broken  cover-glasses  may  be  used  as  supports) 
so  as  not  to  crush  the  animal.     Examine  with  low  power  of  the  com- 
pound microscope.     Notice  that  there  are  two  layers  of  the  body- 
wall  ;  the  outer  (ectoderm)  is  colorless,  the  inner  (endoderm)  is  green 
or  brown.     The  green  is  due  to  the  presence  of  chlorophyll.     In  the 
outer  clear  layer,  look  for  knob-like  swellings,  especially  on  the 
tentacles.     These  swellings  contain  the  stinging  or  nettle-threads, 
which  are  organs  of  defense,  and  also  used  for  spearing  and  paralyz- 
ing water-fleas  and  other  small  water  animals. 

III.  (D  or  L)  Study  of  Sections.  —  Examine  a  longitudinal  section 
of  hydra  with  compound  microscope  and  observe :  — 

The  hydra  is  really  a  hollow  cylinder,  the  interior  of  which  is 
the  digestive  cavity  (Fig.  98) . 

The  tentacles  are  hollow  outgrowths  of  the  digestive  cavity. 

The  mouth  is  the  only  opening  from  this  cavity  to  the  outside. 

The  body-wall  is  composed  of  layers  of  cells  on  both  sides  of 
a  deeply-stained  line  in  the  preparation.  The  outer  layer  is  the 
ectoderm;  this  is  the  colorless  layer  seen  in  the  living  hydra.  The 
deeply-stained  line  (middle  layer)  is  not  made  up  of  cells,  but  con- 
sists of  a  gelatinous  substance.  The  inner  layer  of  large  cells  is  the 
endoderm.  It  contains  the  chlorophyll-bodies  in  green  hydras  and 
the  brown  bodies  in  brown  hydras. 


326 


APPLIED  BIOLOGY 


Look  for  these  three  layers  in  a  section  of  a  tentacle.     In  the 
ectoderm  of  the  tentacles  look  for  the  stinging  cells  or  nettle-cells 

(Fig.  99).  Each  of  these  contains  a 
sac  filled  with  fluid  and  surrounding  a 
coiled  stinging  thread.  If  a  small  ani- 
mal happens  to  touch  a  hair-like  pro- 
jection (trigger)  on  the  outer  edge  of 
one  of  these  cells,  the  stinging  thread 
is  suddenly  ejected  and  spears  the  vic- 
tim. It  appears  that  some  paralyzing 
fluid  is  injected,  for  the  animal  caught 
seems  benumbed.  Finally,  the  ten- 
tacle bends  over  towards  the  mouth 
and  the  captured  food  is  taken  into 
the  digestive  cavity.  The  nettle-cells 
may  also  be  used  for  defense  against 
animals  too  large  for  food  of  a  hydra. 

If  there  is  a  bud  present  on  the  sec- 
tion, notice  the  relation  of  its  digestive 
cavity  to  that  of  the  parent. 

Study  a  transverse  section  of  a  hy- 
dra and  compare  with  the  structure 
in  the  longitudinal 
section. 


FIG.  98.  Longitudinal  section 
of  a  hydra,  t,  tentacle;  m, 
mouth;  e,  ectoderm  or  outer 
layer  of  cells;  n,  endoderm 
or  inner  layer;  d,  digestive 
cavity;  6,  buds;  s,  spermary; 
o,  ovary.  (From  Parker.) 


283.  Physiology 
of  Hydra.  —  The 

life-activities  of  Hydra  are  slightly  more 
specialized  than  those  of  sponge-animals, 
but  they  are  exceedingly  simple  as  compared 
with  those  of  an  animal  like  the  frog. 

Digestion.  —  Small  animals  caught  by  the 
tentacles  and  forced  into  the  digestive  cavity 
through  the  mouth  are  softened  and  disin- 
tegrated by  the  action  of  the  digestive  juice 
secreted  by  the  cells  of  the  endoderm  which 
lines  the  cavity.  The  digestible  particles  set 
free  by  the  disintegrative  action  of  the  digestive  juice  are 
taken  in  by  endoderm  cells,  the  free  ends  of  which  are 


FIG.  99.  a,  sting- 
ing cell  with 
thread  inclosed; 
b,  thread  dis- 
charged. Those 
on  the  tentacles 
of  a  hydra  are 
similar.  (From 
Hatschek.) 


THE  SIMPLEST  MANY-CELLED  ANIMALS          327 

amoeba-like  and  able  to  take  in  food  in  the  way  that  an 
amoeba  does.  Particles  thus  taken  into  the  cells  are  com- 
pletely digested  (dissolved).  This  is  intra-cellular  digestion 
(i.e.,  within  cells),  and  is  the  same  as  the  only  possible 
digestion  in  Protozoa  and  Porifera.  But  in  Hydra  there  is 
a  decided  advance  over  the  lower  animals  in  that  the  cells 
able  to  digest  intra-cellularly  are  grouped  so  as  to  form  the 
lining  of  a  cavity  (digestive  cavity)  into  which  they  pour 
secretions  that  cause  digestion  outside  of  the  cells.  This 
digestion  in  a  cavity,  as  in  the  stomachs  of  all  higher  ani- 
mals, is  extra-cellular  digestion,  i.e.,  occurs  outside  of  cells. 
The  digestive  cells  in  Hydra,  then,  in  part  resemble  the  one- 
celled  animals  and  the  digestive  cells  of  sponge-animals ;  and 
in  part  the  lining  cells  of  the  stomach  of  higher  animals. 

Absorption  of  Digested  Food.  —  The  cells  (endoderm)  which 
line  the  digestive  cavity  of  a  hydra  can  either  get  food  by 
taking  in  particles  and  digesting  them  inside  of  cells,  or  by 
absorbing  foods  digested  in  the  cavity.  The  cells  of  the  outer 
layer  (ectoderm)  must  get  all  their  food  by  absorption  or 
osmosis  from  the  endoderm  cells.  This  osmosis  of  food 
from  cell  to  cell  is  as  easily  accomplished  as  water  soaks 
through  several  layers  of  paper. 

Use  of  food  in  cells  of  hydra  is  the  same  as  in  all  other 
animal  cells.  Some  food  is  used  to  make  new  protoplasm  for 
repair  and  growth,  and  some  serves  as  a  source  of  energy. 

Oxygen  is  absorbed  by  all  cells  in  contact  with  water.  This 
includes  the  majority  of  the  cells,  for  even  the  digestive 
cavity  contains  water  taken  in  with  the  food.  But  probably 
the  larger  part  of  the  oxygen  needed  is  absorbed  from  the 
water  by  ectoderm  cells. 

No  Circulatory  Organs.  —  Although  Hydra  is  multicellular, 
it  is  able  to  live  without  circulating  blood  and  lymph  for  the 
reason  that  its  simple  cylindrical  body  allows  digested  food 
and  oxygen  to  reach  all  cells  by  osmosis,  and  all  excretions 
are  eliminated  by  the  same  process.  Compare  this  with 


328  APPLIED  BIOLOGY 

the  distribution  of  digested  food,  oxygen,  and  excretions  in 
a  frog  (§  52). 

Excretion  is  accomplished  by  osmosis  of  waste  matters  from 
the  cells  to  the  surrounding  water.  Probably  the  ectoderm 
cells  do  the  larger  share  of  this  work. 

Movement  is  caused  partly  by  contraction  of  cells  and  partly 
by  that  of  muscle-processes  which  are  extensions  of  some  cells 
in  both  inner  and  outer  layers. 

Irritability  is  well  developed  in  Hydra.  It  is  in  part  a 
function  of  all  the  constituent  cells ;  but  some  cells  believed 
to  be  simple  nerve-cells  are  near  the  nettle-cells,  which  must 
be  extremely  sensitive  in  order  to  discharge  the  nettle-threads 
when  stimulated  by  other  small  animals. 

Reproduction  of  Hydra  is  both  asexual  and  sexual.  When 
there  is  plenty  of  food  and  other  favorable  conditions,  asexual 
budding  occurs.  A  small  elevation  appears  on  the  side  of  a 
hydra,  and  this  soon  grows  into  a  small  hydra  which  be- 
comes detached  and  lives  independently.  When  the  animals 
are  growing  rapidly,  there  may  be  several  buds  on  one 
animal  at  the  same  time  (Fig.  97). 

Under  certain  unfavorable  conditions,  such  as  lack  of  food, 
stagnation  of  water,  etc.,  hydras  form  reproductive  organs,  — 
ovaries  and  spermaries.  These  appear  as  transparent 
thickenings  of  the  ectoderm,  the  spermaries  near  the  tentacles, 
the  ovaries  near  the  base  of  the  animal.  Any  individual 
may  form  both  kinds  of  sex-organs;  that  is,  each  is  both 
male  and  female;  but  usually  both  kinds  do  not  mature 
on  a  given  individual  at  the  same  time.  An  animal  which 
can  develop  both  ovaries  and  spermaries  is  known  as  her- 
maphroditic. Likewise  a  flower  with  both  stamens  and  pistil 
is  sometimes  called  hermaphroditic.  Sponges,  many  worms, 
many  snails,  and  some  other  lower  animals  also  exhibit 
hermaphroditism. 

When  the  spermaries  of  a  hydra  are  mature,  the  sperm-cells 
escape  and  swim  freely  in  the  water.  A  single  ovum  or  egg- 


THE  SIMPLEST  MANY-CELLED  ANIMALS          329 

cell  develops  in  an  ovary,  and  is  fertilized  by  a  single  sperm- 
cell.  The  fertilized  egg-cell  divides  into  a  large  number  of 
smaller  cells,  forming  an  embryo,  and  becomes  surrounded 
by  a  hard  shell  or  cyst.  The  protected  embryo  falls  to  the 
bottom  of  the  pond.  There  it  may  remain  for  some  time. 
Dry  dust  scraped  from  bottoms  of  ponds  during  mid-summer 
drought  may  contain  embryos  which  will  develop  into  hydras 
soon  after  being  placed  in  water. 

Hydras  have  another  reproductive  process  which  occurs 
only  when  by  some  accident  an  individual  is  cut  into  two 
or  more  pieces.  The  remarkable  fact  is  that  within  a  few 
days  each  piece  will  grow  into  a  perfect  hydra.  This  is  an 
example  of  regeneration.  Many  other  lower  animals  have  the 
same  power  of  forming  a  perfect  body  from  a  part,  and  some 
animals  as  high  as  frogs  can  regenerate  small  parts,  such  as 
toes,  if  they  happen  to  be  destroyed.  It  should  be  noted  that 
regeneration  is  not  a  regular  reproductive  process  in  Hydra, 
but  provides  against  accidents. 

284.  Division  of  Labor  in  Hydra.  —  All  cells  must  use 
food,  but  only  the  endoderm  cells  are  able  to  digest  food. 
Hence,  ectoderm  cells  must  depend  upon  the  endoderm  cells 
for  their  food.     On  the  other  hand,  the  ectoderm  cells  form 
the  external  protective  covering  of  the  animal,  furnish  the  cells 
which  form  the  reproductive  organs,  catch  the  food,  receive 
impressions  from  the  environment,  and  probably  are  the  chief 
causes  of  the  movements  of  the  animal.     The  tentacles  are 
specialized   for   collecting   food,  and   their  nettle-cells  still 
more  highly  adapted  to  a  special  purpose.     The  presence  of 
simple  nerve-cells  suggests  the  beginning  of  differentiation  of 
nervous  organs. 

285.  A   Case   of   Mutual   Aid :     Symbiosis.  —  The   green 
bodies  in  the  endoderm  cells  of  one  species  of  Hydra  have 
been  discovered  to  be  simple,  one-celled  plants  (one  of  the 
Algae),  which  also  live  in  tissues  of  some  fresh-water  sponge 
colonies  and  in  some  other  animals.     Such  a  living  together, 


330  APPLIED  BIOLOGY 

known  as  symbiosis,  of  a  plant  and  animal  is  a  mutually 
helpful  condition.  The  green  plants  use  carbon  dioxide,  an 
excretion  of  the  animal,  and  make  carbohydrate  food,  some 
of  which  may  be  absorbed  as  food  by  the  surrounding  proto- 
plasm of  the  animal.  The  oxygen  freed  from  the  carbon 
dioxide  (§  105),  when  that  is  used  by  the  plant,  may  be  of 
use  to  the  animal's  cells.  The  green  plant  needs  some  nitrog- 
enous food,  and  may  get  this  from  the  animal's  excretions. 
Thus  the  plant  gets  all  the  materials  for  its  food  from  the 
animal,  and  the  animal  gets  carbohydrate  food  made  by 
the  plant  from  the  animal's  carbon  dioxide  excretion.  Here 
within  a  single  animal  we  find  an  illustration  of  the  relation 
which  exists  between  all  animals  and  plants  (§  115). 

286.  Allies  of  Hydra :    Ccelenterates.  —  The  Hydra  is  a 
member  of  a  great  group,  a  primary  division  of  the  animal 
kingdom,  known  as  Coelenterata,  or  popularly  called  ccelen- 
terates.    The  name  is  a  combination  of  ccelome  (the  technical 
name  for  body-cavity  in  all  animals),  and  enteron  (technical 
name  for  a  digestive  cavity),  and  was  originally  given  be- 
cause it  was  supposed  that  in  Hydra  and  its  allies  the  one 
cavity  (the  digestive  cavity)  appeared  to  combine  both  the 
digestive   cavity  and  the   body-cavity  of  higher   animals. 
This  was  wrong,  for  in  the  frog  the  body-cavity  is  between 
the  digestive  cavity  (stomach  and  intestine)  and  the  body- 
wall.     The  cavity  in  Hydra  simply  corresponds  to  the  interior 
of  the  frog's  stomach  and  intestine,  and  if  it  had  a  space 
between  its  endoderm  and  ectoderm  this  space  would  corre- 
spond to  the  body-cavity  in  a  higher  animal. 

The  most  characteristic  features  of  the  ccelenterates  are 
shown  in  Hydra;  namely,  two  layers  of  cells,  a  digestive 
cavity,  and  nettle-cells.  Animals  of  other  groups  have 
tentacles,  but  only  ccelenterates  have  nettle-cells  on  their 
tentacles. . 

287.  Hydroids.  —  Imagine  the  buds  on  a  hydra  remaining 
attached,  as  do  buds  on  trees,  and  budding  repeatedly,  and 


THE  SIMPLEST  MANY-CELLED  ANIMALS 


331 


the  result  woujd  be  a  tree-like  colony  with  a  hydra-like 
animal  at  the  end  of  each  branch.  This,  of  course,  is  im- 
possible for  a  hydra,  for  its  offspring,  formed  from  buds,  always 


FIG.  100.  Hydroids.  a,  colony  consisting  of  root-like  base,  stem,  and  a 
hydranth  at  end  of  each  branch.  6,  piece  of  a  branch  with  two  hy- 
dranths  (hyd)  and  two  medusa  buds  (med)  forming,  c,  a  free  medusa. 
d,  swimming  larva  developed  from  an  egg  produced  by  a  medusa,  e,  f, 
0,  stages  in  development  of  a  larva  into  an  attached  hydroid  which  by 
budding  will  form  a  colony  like  that  shown  in  a.  (From  Parker.) 

separate  quickly;  but  there  are  near  relatives  of  Hydra 
which  from  one  individual  form  a  tree-like  colony  by  such 
repeated  budding.  These  animals  are  called  hydroids, 
which  means  hydra-like.  The  colonies  often  grow  several 
inches  in  height,  with  hundreds  of  branches  and  individual 


332 


APPLIED  BIOLOGY 


hydroids.  One  who  does  not  examine  with  a  band-lens  would 
probably  mistake  a  group  of  hydroid  colonies  on  a  rock  or 
other  object  for  a  mass  of  moss-like  plants  (Figs.  100, 101, 102). 

(D)  Various  species  of  hydroids  can  be  obtained  from  dealers 
in  biological  supplies.  They  are  best  preserved  in  formalin.  Single 
colonies  can  be  picked  out  from  a  mass 
and  placed  in  small  glass  vials  for  ex- 
amination with  a  hand-lens.  Some  of 
the  individual  hydroids  (polyps  or  hy- 
dranths)  should  be  mounted  in  water  or 
glycerin  for  examination  with  low  power. 
Some  stained  permanent  preparations  of 
entire  hydranths  and  sections  will  be 
useful. 

Observe  :  (1)  general  form  of  colony; 
(2)  position  of  individuals  ;  (3)  the  sup- 
porting stem  or  stalk  (which  has  the  same 
layers  as  in  Hydra  plus  a  transparent 
outer  sheath  of  a  hardened  material  which 
is  protective  and  at  the  same  time  affords 
an  elastic  support  to  the  colony) ;  (4) 
hydra-like  form  of  the  individuals  (ten- 
tacles, mouth,  nettle-cells,  digestive  cav- 
ity, two  layers  of  cells,  digestive  cavity 
continued  into  the  hollow  stalk).  In 
some  species  of  hydroids  the  transparent 
sheath  of  the  stalk  stops  at  the  base  of 
each  individual  polyp,  but  in  others  it  expands  to  form  a  transparent 
cup  for  each  polyp. 


FIG.  101.  A  colony  of  Hy- 
dractinia  hydroids  with 
differentiated  nutritive 
(ri),  defensive  (d),  and  re- 
productive (r)  individu- 
als which  co-operate.  The 
digestive  cavity  of  n  con- 
nects with  that  of  d  and  r 
which  are  unable  to  take 
food.  (FromMcMurrich.) 


288.  Medusae.  —  Some  species  of  hydroid  colonies  have 
certain  polyps  of  a  bell-shaped  form  (Fig.  100,  6,  c),  and  in 
other  species  there  are  special  polyps  which  are  elongated 
and  without  tentacles.  Those  of  the  bell-form  break  away 
from  the  hydroid  stem  and  swim  freely  as  medusae  or  jelly- 
fishes.  Those  of  the  elongated  type  form  a  number  of  buds, 
and  each  bud  develops  into  a  bell-shaped  medusa.  All  the 
medusae  are  very  small  when  first  freed,  but  they  have 
mouth  and  tentacles  and  are  able  to  catch  food,  as  a  hydra 


THE  SIMPLEST  MANY-CELLED  ANIMALS 


333 


does,  and  grow  rapidly.  Some  of  them  may  grow  to  be 
several  inches  in  diameter.  The  structure  of  a  typical 
medusa  should  be  observed. 

(D)   Specimen  of  a  medusa  known  as  Gonionemus  may  be  pur- 
chased from  the  laboratory  at  Woods  Hole,  Mass.     They  may  be 


a       b 


FIG.  102.  A  colony  of  hydroids  growing  on  a  mussel  shell.  The  erect 
branches,  a,  6,  c,  d,  show  stages  in  development  of  branches  by  budding. 
The  entire  colony  originated  from  a  single  egg  developed  into  a  free- 
swimming  larva  which  attached  to  the  shell  and  produced  the  colony 
by  budding.  (After  Schulze.) 

kept  in  small  bottles  for  convenience  in  demonstrations,  or  be  placed 
in  water  in  watch-crystals.  Notice:  (1)  bell-form;  (2)  tentacles; 
(3)  mouth  at  end  of  the  central  projection,  which  is  in  same  position 
as  clapper  of  a  bell ;  (4)  digestive  canals  forming  a  conspicuous  cross 
and  a  ring  around  the  margin  of  the  bell,  and  connecting  with 
the  mouth  as  may  be  seen  in  a  specimen  into  whose  mouth  some 
pigment  in  water  has  been  injected  with  a  pipette;  (5)  irregular 


334  APPLIED  BIOLOGY 

bodies  (ovaries  or  spermaries)  are  attached  inside  the  bell  and  just 
beneath  the  digestive  canals ;  (6)  a  muscular  membrane  around  the 
inner  margin  of  the  bell  is  responsible  for  the  locomotor  movements 
of  the  medusa  through  the  water.  See  Figure  100,  c. 

A  medusa  is  more  highly  specialized  than  a  polyp  of  a 
hydroid  colony,  for  in  addition  to  the  nutritive  functions  of 
the  polyp,  the  medusa  has  a  muscular  organ  for  swimming,  a 
ring  of  nerve-cells  with  their  fibers,  and  the  reproductive 
organs. 

Embryos.  —  Egg-cells  and  sperm-cells  produced  by  medusae 
result  in  fertilized  eggs,  which  develop  into  free-swimming 
embryos.  Each  embryo  after  a  time  settles  down  on  rocks, 
piles,  and  other  objects,  and  becomes  a  hydra-like  polyp 
which  looks  much  like  one  of  the  ordinary  polyps  seen  in 
colonies.  This  grows  and  forms  buds,  and  by  repeated  bud- 
ding a  new  hydroid  colony  is  formed. 

289.  Alternation  of  Generations. — The  above  account 
makes  it  evident  that  the  hydroid  colony  and  the  medusae 
alternate  just  as  do  the  fern  plant  and  the  prothallium  (§  226). 
The  hydroid  colony  forms  ordinary  polyps  and  medusae 
by  budding  (an  asexual  reproduction) ;  the  medusae  produce 
germ-cells  (ova  and  sperm-cells),  which  start  new  hydroids 
able  to  bud  and  form  colonies.  The  colony  is'  the  asexual 
generation,  the  medusa  is  the  sexual  generation.  In  very 
many  species  of  hydroids  this  alternation  of  generations 
is  just  as  necessary  a  part  of  the  life-cycle  as  it  is  in  ferns  and 
mosses.  Medusae  of  such  species  cannot  produce  eggs 
which  are  able  to  develop  into  medusae  without  the  hydroid 
colony  stage;  and  conversely,  the  hydroid  colony  cannot 
form  new  colonies  without  the  medusa  stage. 

There  are  some  species  of  medusae  closely  related  to 
those  formed  by  hydroids  whose  eggs  develop  directly  into 
new  medusae ;  that  is,  these  species  do  not  have  alternation 
of  generations. 

In  one  of  the  common  hydroids  of  the  New  England  coast 


THE  SIMPLEST  MANY-CELLED  ANIMALS 


335 


the  medusae  sometimes  do  not  develop  enough  to  become 
free-swimming,  and  remain  attached  to  the  colony.  However, 
they  produce  embryos  in  the  usual  way,  and  the  embryos 
swim  away  and  start  new  colonies. 

290.  The  Larger  Jelly-fishes.  —  The  term  "  jelly-fish  " 
usually  applies  to  some  of  the  large  medusae,  some  of  which 
are  from  five  to  eight  feet 
in  diameter.  Their  general 
structure  resembles  that  of 
the  medusae  described  above ; 
but  the  life-history  does  not 
include  a  hydroid  colony. 
Instead,  an  egg  produced 
by  the  large  medusae  develops 
into  a  ciliated  embryo,  which 
swims  for  a  time,  settles  down 
and  changes  into  a  small 
animal  looking  like  a  hydra. 
This  divides  transversely  into 
a  number  of  disks  so  that  the 
animal  now  looks  like  a  pile 
of  saucers.  Each  of  these 
saucer-shaped  bodies  becomes 
a  medusa,  and  grows  rapidly, 
of  generations.  The  large  size  of  the  adults  is  partly  due  to 
absorption  of  water,  more  than  nine  tenths  of  a  jelly-fish 
being  water.  See  Fig.  103. 

The  Ctenophores  or  comb-jellies,  named  because  of  eight 
bands  of  comb-like  appearance,  consisting  of  vibrating  plates 
which  cause  locomotion,  are  the  highest  ccelenterates. 
Museum  specimens  should  be  examined.  The  larger  ones 
are  often  the  cause  of  the  phosphorescent  masses  seen  in 
sea-water  when  a  boat  is  being  paddled  on  a  dark  mid-summer 
night.  Ctenophore  eggs  develop  into  young  ctenophores; 
there  is  no  alternation  of  generations. 


FIG.  103.  Development  of  one  of 
the  larger  species  of  jelly-fishes. 
1,  larva  developed  from  an  egg;  2, 
3,  4,  5,  stages  in  development  of 
larva  into  a  fixed  hydra-like  or- 
ganism; 7,  8,  9,  division  into  many 
disks,  each  forming  a  young  me- 
dusa (11,  12).  (From  Hatschek.) 

There  is,  then,  alternation 


336 


APPLIED  BIOLOGY 


291.  Coral-animals.  —  Most  important  to  man  of  all  the 
allies  of  Hydra  are  the  animals  whose  skeletons  form  a  large 
part  of  many  coral  islands.  (Look  in  a  textbook  of  geography 
for  a  list  of  coral  islands.)  In  order  to  understand  how  coral- 
animals  form  their  hard  skeletons,  it  is  necessary  to  examine 
a  specimen  of  a  sea-anemone;  these  resemble  coral-animals 

in   structure,    but    do  not 
secrete  skeletons. 

(D)  Living  sea-anemones  are 
attached  to  rocks  and  piles  be- 
low low-tide  mark.  Schools 
near  the  sea-shore  could  keep 
small  ones  in  a  salt-water 
aquarium ;  but  most  schools 
must  depend  upon  museum 
specimens  preserved  in  forma- 
lin. An  entire  sp'ecimea,  one 
split  longitudinally,  and  one 
cut  transversely,  will  show  the 
essential  points  of  structure. 

In  an  entire  sea-anemone, 
notice  :  the  cylindrical  body, 
the  base,  the  crown  of  small 
tentacles,  and  the  mouth  in 
the  center  of  the  crown  of 
tentacles. 

In  a  sea-anemone  which  has 

been  split  lengthwise,  notice :  the  mouth  opening  into  a  long  tube 
(esophagus)  extending  down  to  near  the  base,  and  the  delicate 
membranes  connecting  this  tube  with  the  body-wall. 

In  a  specimen  cut  transversely,  notice  :  the  esophagus  in  the  center, 
the  pairs  of  membranes  (mesenteries)  which  extend  from  the  esopha- 
gus to  the  body-wall,  and  the  pairs  of  shorter  membranes  which 
extend  inwards  a  short  distance  from  the  wall.  All  the  spaces  be- 
tween the  membranes  extend  up  into  the  tentacles  and  down  to 
the  opening  of  the  esophagus  (compare  with  the  longitudinal  section). 
Foods  taken  in  through  the  esophagus  may  pass  anywhere  between 
the  membranes,  and  be  digested.  In  brief,  the  digestive  cavity  of 
a  sea-anemone  is  more  complex  than  that  of  a  hydra  in  that  it  is 
subdivided  by  many  pairs  of  membranous  partitions. 


FIG.  104.  Common  sea-anemone  con- 
sisting of  a  cylindrical  body  with  a 
crown  of  numerous  small  tentacles 
surrounding  the  mouth.  (After  Em- 
erton.) 


THE  SIMPLEST  MANY-CELLED  ANIMALS          337 

There  are  many  interesting  minor  points  of  structure  omitted  from 
these  brief  notes.  For  supplementary  work,  consult  Linville  and 
Kelly's  "General  Zoology,"  Chapter  XIX. 

Formation  of  Coral  Skeletons.  —  On  every  piece  of  coral 
which  has  not  been  corroded  by  water,  there  may  be  seen 
numerous  cup-like  depressions,  each  with  many  radial 
partitions.  In  each  of  these  cups  there  was  once  seated  a 
coral-animal,  or  coral-polyp,  which  secreted  the  cup.  Im- 


FIG.  105.  An  Astraea  coral  colony,  with  living  animals  on  surface  of  hemi- 
spherical skeleton.  Such  a  colony  originates  from  a  single  egg  which 
develops  into  a  polyp  capable  of  budding  to  form  a  colony.  (After  Dana.) 

agine  a  sea-anemone  able  to  secrete  a  calcareous  skeleton 
around  itself,  and  also  to  secrete  a  partition  of  the  same 
material  between  the  two  membranes  of  each  pair  seen  in  a 
transverse  section,  and  then  you  can  understand  the  relation 
of  a  coral-animal  to  its  surrounding  cup. 

The  fact  that  a  piece  of  coral  shows  many  cups  is  explained 
by  the  multiplication  of  individuals  by  budding.  A  coral- 
animal  which  develops  from  a  fertilized  egg-cell  settles 
down  on  a  rock  or  on  skeletons  of  pre-existing  corals  and  begins 
to  secrete  a  skeleton  around  itself.  Buds  are  formed,  which 
do  not  become  detached  as  in  Hydra,  and  these  begin  to 


APPLIED  BIOLOGY 


secrete  skeletons.  The  final  result  of  oft-repeated  budding 
is  a  complicated  calcareous  mass,  in  tree-like  or  hemispheri- 
cal form,  with  numerous  cups  representing  the  number  of 
polyps  which  took  part  in  the  formation. 

The  sea-fans  and  sea-plumes  are  coral  skeletons  formed 
in  another  way.  Their  outer  surfaces  are  calcareous,  and 
their  central  axis  is  composed  of  a  horny  material.  This  is 

black  in  many  species,  but  red 
in  the  case  of  the  precious  coral 
from  the  Mediterranean,  which 
is  used  in  jewelry. 

One  species  of  coral-animal  is 
found  in  Long  Island  Sound,  but 
most  of  them  require  warmer 
waters.  They  can  live  at  any 
depth  down  to  about  300  feet, 
but  are  seldom  found  below  120 
feet.  They  must  have  clean 
and  undiluted  sea-water.  Most 
commonly  the  young  animals 
developed  from  eggs  attach 
themselves  to  favorable  sea- 
bottom  near  land,  and  form 
fringing  reefs  of  coral  rock  near 
the  shore,  or  barrier  reefs  when 
a  navigable  channel  is  eroded  between  the  reef  and  the 
shore.  The  formation  of  an  atoll,  a  peculiar  coral  island 
in  ring  form  and  inclosing  a  body  of  water,  may  have 
begun  as  a  fringing  or  barrier  reef  around  a  volcanic 
island  which  has  later  subsided  or  been  eroded  away  while 
the  deposits  of  coral  rock  have  been  accumulating.  Or  a 
small  group  of  corals  may  have  been  established  on  sea- 
bottom  at  a  favorable  depth,  and  as  the  colony  grew  out- 
wards, the  animals  in  the  center  may  have  been  killed  by 
coral-sand  washed  over  them  by  the  waves.  Erosion  takes 


FIG.  106.  A  colony  of  coral-ani- 
mals whose  supporting  "skele- 
ton "  (black  in  figure)  contairite 
the  red  coral  of  commerce. 
(After  Lacaze-Duthiers.) 


THE  SIMPLEST  MANY-CELLED  ANIMALS          339 

place  rapidly  when  there  are  no  living  coral-animals  to  keep 
adding  to  the  coral  rock ;  and  so  the  central  rock  might  be 
worn  away,  leaving  a  ring  of  living  animals  continually 
adding  new  coral  rock  to  the  outside. 

292.  Economic  Relation  of  Coelenterates.  —  There  are 
probably  more  than  three  thousand  species  of  ccelenterates, 
but  with  the  exception  of  coral-animals,  none  of  them  are 
of  direct  economic  importance.  Some  of  them  serve  as  foods 
for  animals  of  the  sea,  e.g.,  small  fishes  may  be  seen  biting  oft7 
the  branches  of  hydroid  colonies;  but  the  large  amount  of 
water  in  the  larger  forms  makes  them  of  little  food  value 
to  other  animals.  However,  the  ccelenterates  prove  that 
human  interest  is  not  limited  simply  to  those  things  which 
have  a  money  value  or  are  good  to  eat,  for  the  various  forms 
of  hydroids,  jelly-fishes,  comb-jellies,  and  sea-anemones  have 
always  been  great  favorites  with  both  amateur  and  profes- 
sional naturalists.  The  great  importance  of  the  coral- 
animals  is,  of  course,  that  of  forming  islands  and  reefs  which 
protect  the  shores  of  other  islands.  Also,  there  is  the  red 
coral,  already  mentioned  as  valuable  for  ornaments. 

Classes  of  Ccelenterata 

Hydrozoa  —  Hydra,  hydroids,  small  medusa. 
Actinozoa  —  sea-anemone,  coral-animals. 
Scyphozoa  —  large  medusae  (jelly-fishes). 
Ctenophora  —  ctenophores   (comb-jellies). 


CHAPTER  XII 
THE    WORM-LIKE   ANIMALS 

293.  The  Worms. —The  word 

than  a  name  for  a  shape,  meaning  an  elongated  animal  with 
cylindrical  or  ribbon-like  body.  The  earlier  naturalists 
thought  the  shape  very  important,  and  so  they  placed  all  worm- 
like  animals  together  in  a  group  which  they  named  Vermes 
(Latin  for  worms) ;  but  later  studies  have  shown  that  worm- 
like  bodies  may  belong  to  animals  which  in  all  other  points 
of  structure  are  seen  to  be  totally  unrelated.  For  this  reason, 
the  group  Vermes  is  not  recognized  in  the  recent  textbooks 
of  zoology;  but  instead  we  find  chapters  devoted  to  such 
groups  as  flat  worms,  round  worms,  segmented  worms,  and 
others.  There  is  such  a  vast  assemblage  of  forms  of  the 
worm-like  animals  that  in  a  short  course  we  can  do  no  more 
than  select  a  few  of  the  more  interesting  examples. 

294.  The  Flat  Worms.  —  These  are  the  simplest  worm- 
like  animals,  and  some  of  them  are  but  slightly  more  complex 
than  some  of  the  highest  ccelenterates.     Some  of  the  common 
species,    known   as   planarians,    are   brown   in   color,    from 
J  to  \  inch  long,  and  very  easily  found  on  the  under  side  of 
stones  in  fresh  water  streams  and  ponds.     Small  leeches 
resemble  them,  but  the  leeches  are  easily  distinguished  by 
the  numerous  rings  around  their  bodies.     The  general  appear- 
ance of  a  common  planarian  is  shown  in  Fig.  107. 

295.  Other  Flat  Worms  :   Tape- Worms. — All  the  ribbon- 
like  worms  known  as  tape-worms  are  parasites  in  the  ali- 
mentary canal  of  vertebrate  animals.     They  are  simpler  than 
the  flat  worms,  which  live  independently,  for  they  have  no 

340 


THE   WORM-LIKE  ANIMALS 


341 


mouth  or  digestive  organs,  but  absorb  through  their  skin  the 
digested  food  in  the  alimentary  canal  of  the  host  in  which 
they  live. 

A  tape- worm  preserved  and  mounted  in  a  jar  containing 
alcohol  or  formalin  will  show  that  the  body 
resembles  a  long,  narrow  ribbon  or  tape, 
but  is  unlike  a  ribbon  in  that  it  is  divided 
into  segments.  The  ribbon  narrows  near 
the  anterior  end.  The  head  is  a  rounded 
knob,  with  a  circle  of  hooks  and  four 
suckers.  There  are  nerves  and  excretory 
tubes  in  each  segment.  Each  of  the  larger 
segments  towards  the  posterior  end  has  a 
complete  hermaphroditic  reproductive  sys- 
tem, consisting  of  ovaries,  spermaries,  and 
their  ducts  leading  to  the  surface.  The 
extreme  posterior  segments  become  greatly 
distended  with  enormous  quantities  of  fer- 
tilized eggs,  each  inclosed  in  a  hard  shell  ; 
and  one  by  one  these  "  ripe  "  segments 
drop  off  and  are  carried  out  of  the  intes- 
tine with  indigestible  matters  or  feces.  As 
segments  drop  off  the  worm,  new  ones  are 
formed  by  new  grooves  in  the  segments 
back  of  the  head.  Thus  the  oldest  seg- 
ments are  the  most  posterior  ones. 

At  the  time  a  "  ripe  "  segment  is  dis- 
charged from  the  intestine,  the  egg-shell 
contains  a  small  embryo  with  six  hooks. 
If  these  small  embryos,  in  the  case  of  the 
human  tape-worms,  happen  to  fall  on  grass  or  other  food  of 
pigs  and  cattle,  the  digestive  fluids  in  the  stomachs  of  these 
animals  can  dissolve  the  hard  shell  and  free  the  embryo. 
Then  it  bores  into  some  organ  and  becomes  encysted.  Then 
it  develops  a  bladder-like  structure  with  a  tape- worm  head. 


FIG.  107.  A  plana- 
rian,  flat  worm. 
The  root-like  in- 
testine is  shown  in 
black,  e,  eye;  p, 
pharynx;  ra,  mou!h. 
Delicate  excretory 
tubes  lie  on  either 
side  of  the  body. 
(From  Hatschek.) 


342 


APPLIED  BIOLOGY 


This  stage  (called  a  bladder-worm  or  cysticercus)  remains 
for  some  time  in  the  tissues;  and  if  uncooked  pork  or  beef 
be  eaten  by  man,  an  encysted  bladder-worm  may  attach  itself 
to  the  wall  of  the  intestine  by  means  of  its  hooks  and  suckers 

and  develop  into  a 
tape-worm. 

The  above  ac- 
count of  the  human 
tape-worms,  of 
which  one  species 
develops  its  larval 
stage  in  pigs  and 
another  in  cattle, 
represents  the  life- 
history  of  all  the 
tape-worms  which 
inhabit  the  intes- 
tines of  various  ver- 
tebrates. Two  hosts 
are  required,  one  for 
the  tape-worm,  and 
one  for  the  interme- 
diate bladder-worm 
stage.  One  species 
of  tape-worm  of 
dogs  has  its  bladder- 
worm  in  rabbits ; 
one  of  sea-gulls  has 
its  in  earth-worms ;  and  one  of  cats  has  its  in  liver  of  rats 
and  mice.  In  Oriental  countries  there  is  a  large  human 
tape-worm  whose  bladder-worm  stage  is  passed  in  certain 
fishes.  In  a  large  number  of  cases,  the  bladder-worm  stage 
occurs  in  the  particular  animal  which  is  a  favorite  food  of 
the  animal  which  may  be  the  host  of  the  fully  developed 
worm. 


FIG.  108.  Tape-worm,  a,  head;  b,  larval  stage 
found  in  flesh;  c,  parts  of  an  adult  worm  from 
intestine.  (From  Hatschek.) 


THE   WORM-LIKE  ANIMALS  343 

Tape-worms  are  troublesome  parasites  in  that  they  inter- 
fere with  the  nutrition  of  the  animals  which  they  inhabit. 
They  are  difficult  to  remove,  because  the  head  is  so  firmly 
attached  to  the  lining  of  the  intestine.  All  the  segments 
might  be  dislodged  by  powerful  drugs,  but  if  the  head  re- 
mained, it  might  continue  to  grow  new  segments. 

The  bladder-worms  embedded  in  human  tissues  cause  an 
uncommon  disease  known  as  hydatids. 

Prevention  of  tape- worms  is  a  simple  matter;  namely,  eat 
no  meat  "  cooked  very  rare."  In  the  case  of  pork  there  is  an 
additional  reason  for  this  rule,  in  that  the  far  more  dangerous 
parasite  Trichina  (§  297)  may  be  present.  With  increasing 
attention  to  sanitation  and  the  disposal  of  sewage  by  bacterial 
methods  described  in  §  258,  /,  there  will  be  less  chance  of  tape- 
worm embryos  getting  into  pigs  and  cattle  and  then  indirectly 
entering  human  beings.  The  rarity  of  human  tape-worms 
in  United  States  is  probably  in  part  due  to  the  fact  that  our 
farms  are  more  sanitary,  and  being  larger  than  those  of  Europe 
farm  animals  live  farther  away  from  human  dwellings. 

296.  Other  Parasitic  Flat  Worms.  —  One  of  the  best 
known  flat  worms  whose  habits  of  life  resemble  those  of  the 
tape-worm  is  the  liver-fluke,  which  lives  in  the  bile-ducts  of 
sheep.  It  is  a  flat  worm  about  one  inch  long  and  one  fourth 
inch  broad,  with  two  suckers  for  attachment.  The  eggs 
escape  into  the  sheep's  intestine  through  the  bile-duct,  and 
after  being  discharged  from  the  intestine,  an  egg  develops 
into  a  larva  covered  with  cilia.  This  larva  swims,  and  if  it 
reaches  a  pond-snail,  bores  into  it  and  becomes  an  elongated 
sac.  Inside  this  sac  are  formed  many  new  larvae,  and  inside 
each  of  these  larvae  are  formed  many  more.  Thus  one  larva 
entering  a  snail  produces  a  large  number  of  larvae  of  different 
forms;  and  each  one  of  these  may  leave  the  snail,  become 
encysted  on  grass,  and  when  eaten  by  a  sheep  will  enter  the 
bile-duct  and  develop  into  a  liver-fluke.  When  once  a  damp 
pasture  along  a  given  stream  has  become  infested  with  larvae 


344  APPLIED  BIOLOGY 

of  liver-flukes,  the  only  way  to  avoid  infecting  sheep  is  to 
keep  them  away  from  that  stream  for  a  number  of  years. 
Ultimately  the  fluke  larvae  would  disappear  from  a  pasture 
if  there  were  no  sheep  in  which  the  parasites  could  complete 
the  life-history. 

297.  Round  Worms.  —  A  good  example  of  a  harmless 
round  worm  is  the  vinegar-eel,  which  is  often  abundant  in 
unfiltered  vinegar,  and  especially  in  "  mother  of  vinegar." 
They  should  be  examined  with  the  low  power  of  a  microscope. 
Many  species  of  the  round  worms  are  parasitic  in  intestines 
of  man  and  animals.  Some  are  slender  threads  half  an  inch 
long,  while  others  may  be  a  foot  long. 

The  most  terrible  round  worm  is  the  trichina.  The  adult 
worms  live  in  the  intestine  of  man,  pig,  and  other  mammals. 
The  males  are  about  1  mm.  (^V  inch)  and  the  females  3  mm. 
long.  The  eggs  develop  inside  the  female  and  the  young 
(1000  or  more)  are  born  alive;  that  is,  discharged  as  young 
worms.  These  pass  through  the  walls 
of  the  intestine  of  their  host,  and  reach 
such  muscles  as  those  of  the  arms,  legs, 
and  back.  Each  worm  embeds  itself  in 
a  muscle-fiber  (Fig.  109),  and  a  cyst 
forms  around  it.  It  may  remain  en- 
cysted for  years.  An  ounce  of  infected 
pork  may  have  80,000  such  cysts.  If 
the  flesh  be  eaten  by  another  animal  or 
by  man,  the  cysts  dissolve  in  the  diges- 
FIG.  109.  Trichinae  ^ive  organs,  the  young  worms  develop, 

encysted  in  muscle.      «  ,  ,   -      ,.,.       -, 

(After  Leuckart.)  form  eggs  and  sperms,  and  fertilized  eggs 
develop  into  young  worms  which  become 
encysted  in  muscles.  In  a  town  in  Germany,  in  1884,  the 
flesh  of  one  pig  infected  364  persons,  and  57  died  within  a 
month.  This  large  number  of  infections  is  easily  understood 
when  we  calculate  from  the  figures  above.  A  single  ounce  of 
pork  might  introduce  into  a  human  stomach  80,000  trichinae . 


THE   WORM-LIKE  ANIMALS  845 

Suppose  one  half  of  these  are  females,  each  able  to  produce 
1000  young  worms,  and  four  million  encysted  worms  might 
be  the  result.  Thus,  at  this  rate,  100  pounds  of  pork  might 
contain  enough  cysts  to  develop  64  billions  of  encysted  worms, 
or  160  million  for  each  of  400  persons  who  might  eat  the  pork. 
And  this  is  not  the  whole  story,  for  a  female  trichina  may 
live  in  the  human  intestine  and  frequently  give  birth  to  as 
many  as  1000  young.  It  is  easy  from  such  figures  to  see  how 
a  single  infected  pig  could  have  caused  so  much  trouble. 

There  is  no  way  to  stop  the  worms  when  once  they  get 
into  the  human  intestine.  If  the  infected  individual  does 
not  soon  die  from  the  inflammation  caused  by  the  encysting 
in  the  muscles,  the  cysts  soon  become  hardened  and  there  is 
no  more  danger  to  the  patient.  Prevention  is  very  simple; 
namely,  eat  no  pork  which  is  not  well  cooked.  Government 
inspectors  in  the  United  States  and  other  countries  examine 
meats  at  the  great  packing  houses  and  slaughter  houses,  and 
condemn  as  unfit  for  human  food  all  meat  found  to  have 
trichinae.  Inspection  is  not  difficult,  for  if  the  parasites  are 
present  in  a  pig,  they  are  likely  to  be  so  abundant  that  a  small 
piece  of  lean  meat  (muscle)  examined  with  a  microscope 
will  reveal  the  trichinae.  However,  the  parasites  have 
been  sometimes  overlooked  by  expert  inspectors. 

The  above  account  has  not  explained  how  pigs  get  infected. 
Since  they  do  not  eat  human  flesh,  they  must  get  the  para- 
sites from  some  other  animal.  Trichinae  are  found  in  rats, 
and  it  is  well  known  that  pigs  will  eat  dead  rats.  Also, 
pigs  might  eat  scraps  of  pork  thrown  out  in  garbage. 

298.  The  Horsehair  Worm.  —  In  some  rural  districts,  it 
is  still  believed  that  a  long,  thread-like  worm,  looking  not 
unlike  a  long  hair  from  a  horse's  mane  or  tail,  and  found 
wriggling  in  pools  of  water,  has  developed  from  horsehairs 
which  have  happened  to  fall  into  the  water.  Hence  the 
names  "  horsehair  worm  "  or  "  horsehair  snake."  The  be- 
lief seems  to  have  been  originated,  and  is  still  perpetuated, 


346     -  APPLIED  BIOLOGY 

( 

by  the  fact  that  the  worms  are  sometimes  seen  wriggling  in 
troughs  where  horses  drink,  and  where  horsehairs  may  also 
be  seen.  But  that  there  is  no  connection  between  the  worms 
and  the  hairs  can  be  demonstrated  by  any  one  who  will 
place  in  bottles,  stoppered  with  cotton,  one  or  a  thousand 
horsehairs,  and  await  developments. 

Scientific  men  long  ago  studied  the  structure  and  embry- 
ology of  horsehair  worms  and  solved  the  mystery  of  their 
appearance  in  pools,  watering  troughs,  etc.  The  structure 
of  the  worm  is  essentially  the  same  as  that  of  the  other 
round  worms.  Its  scientific  name  is  Gordius,  in  allusion  to 
its  habit  of  twisting  into  a  tangle  like  the  famous  Gordian 
knot  which  Alexander  the  Great  cut  with  his  sword.  Its 
eggs  develop  into  minute  larvae,  which  become  parasites  in 
insects,  fishes,  frogs,  and  other  animals.  Later  the  parasites 
escape  from  these  animals  and  develop  into  horsehair  worms. 
Those  seen  in  horse-troughs  have  completed  their  life-history 
in  insects. 

299.  Spontaneous  Generation.  —  The  belief  that  a  living 
worm  might  develop  from  a  dead  horsehair  is  opposed  to 
the  commonly  accepted  idea  that  living  things  develop  only 
from  similar  living  things.  However,  this  idea  that  all  life 
comes  from  life  (biogenesis)  is  rather  recent.  In  former  times, 
even  scientific  men  believed  in  spontaneous  generation  of  life 
in  dead,  or  not-living  matter.  The  sudden  appearance  of 
numerous  earthworms,  frogs,  mice,  insects,  etc.,  was  ex- 
plained by  assuming  that  they  had  suddenly  originated  spon- 
taneously. When  accurate  studies  of  life-histories  began  to 
be  made,  it  soon  became  evident  that  all  the  larger  animals 
and  plants  originate  only  from  organisms  like  themselves. 
Until  1638,  it  was  supposed  that  maggots  developed  spon- 
taneously from  putrid  meat ;  but  in  that  year  an  investiga- 
tor showed  that  maggots  never  appear  on  meat  which  is 
screened  so  as  to  keep  flies  from  laying  eggs  on  it.  Studies 
of  habits  have  shown  that  tfce  sudden  appearance  of  earth- 


THE   WORM-LIKE  ANIMALS  347 

worms  is  caused  by  flooding  of  their  burrows,  and  that  they 
do  not  rain  down ;  and  that  crowds  of  toads,  frogs,  mice,  grass- 
hoppers, etc.,  are  due  to  very  favorable  conditions  for  the 
developing  eggs  and  migrations  into  new  territory.  All 
other  such  cases  which  puzzled  even  the  scientific  people  of  a 
few  hundred  years  ago  have  been  explained  so  well  that  for 
more  than  a  hundred  years  no  scientific  man  has  believed  in 
the  existence  of  spontaneous  generation  of  any  organisms 
higher  than  the  bacteria.  But  until  the  studies  by  Pasteur, 
supported  by  the  work  of  the  English  physicist  Tyndall, 
between  1850  and  1870,  it  was  believed  even  by  men  of 
science,  that  certain  bacteria  may  develop  spontaneously  in 
sterile  bouillon  and  other  foods.  Pasteur  showed  that  if 
proper  precautions  are  taken  to  make  the  foods  perfectly 
sterile,  that  is,  to  kill  all  life  in  the  test-tubes  used,  and  to 
prevent  entrance  of  other  germs,  no  organisms  will  develop. 
In  short,  Pasteur  showed  that  there  is  no  evidence  that  liv- 
ing matter  originates  spontaneously  from  not-living  matter. 
He  did  not  show  that  it  could  not  happen,  for  there  may  be 
conditions  of  which  we  know  nothing,  as  perhaps  existed  at 
the  first  appearance  of  living  matter  on  the  earth;  but  he 
showed  that  the  few  cases  in  which  some  scientific  men  of 
his  time  still  believed  had  not  been  sufficiently  tested  by  accu- 
rate experiments.  So  far  as 
concerns  the  origin  of  new  in- 
dividuals of  all  known  species 

Of    Organisms  nOW  existing,   We      FIG.   IIO.     Plan  of  annelid's  body, 
may     summarize      the      Studies          &.  "  brain,"  dorsal  to  the  esoph- 

which  culminated  with  those  of       ££'  £  ^0™*^ 

Pasteur  in    the   Statement  that          alimentary   canal ;    a,    anus    or 

all  living  things  come  from  liv- 
ing  things,  or  all  life  from  life. 

300.  The  Segmented  Worms  :  Annelids.  —  The  common 
earthworms  and  the  leeches  are  members  of  a  group  char- 
acterized by  division  of  the  body  into  rings  or  segments, 


348 


APPLIED  BIOLOGY 


(also  called  metameres).  They  are  much  higher  than  the 
worms  previously  mentioned.  There  is  much  diversity  of 
form  and  habit  among  the  members  of  this  group,  and  we  can- 
not now  take  time  for  more  than  a  brief  examination  of  a  few 

common  examples  of 
the  segmented  worms. 


301.  The  Sand- 
worm. — (D)  This  worm, 
belonging  to  the  genus 
Nereis,  lives  in  sand  at 
the  sea-shore  near  low- 
tide  mark,  and  thousands 
are  dug  and  used  by  fish- 
ermen for  bait.  In  cer- 
tain seasons  they  are 
rapid  swimmers  in  the 
sea.  Specimens  may  be 
kept  and  examined  in 
long  test-tubes,  or  in 
glass  tubing  stoppered  at 
each  end  with  a  cork  and 
then  dipped  into  hot  par- 
affin. Examine  a  sand 
worm  according  to  the 
following  notes.  It  has 
a  head  with  jaws,  small 
tentacles,  two  pairs  of 
bead-like  eyes,  and  a 
pair  of  cylindrical  palps 
(feelers).  On  some  speci- 
mens the  proboscis  may 
be  found  extended  from 
the  mouth.  There  are  several  pairs  of  cirri  (feelers)  back  of  the 
head.  The  body  is  divided  into  segments  ;  count  the  number  in  an 
inch  of  length  and  then  estimate  the  total  number.  Are  the  seg- 
ments similar?  How  are  the  rowing  organs  or  paddles  arranged 
on  the  segments  ?  Perfect  specimens  have  a  forked  tail-appendage. 
The  main  longitudinal  blood-vessels  appear  through  the  skin  on 
dorsal  and  ventral  surfaces. 

Dissected  specimens  pinned  on  pieces  of  soft  wood  or  on  cork  may 


FIG.  111.  Development  of  a  marine  annelid. 
1,  fertilized  egg-cell;  2,  two-cell  stage;  3, 
four-cell  stage;  4,  eight-cell  stage;  5-8,  later 
stages  which  end  in  the  free-swimming  larva, 
(9),  and  this  metamorphoses  (10)  into  the 
adult  form  with  numerous  segments  (11). 
m,  mouth;  a,  anus;  e,  alimentary  canal. 
(From  Thomson,  after  Fraipont.) 


THE   WORM-LIKE  ANIMALS 


349 


be  used  to  demonstrate  the  presence  of  digestive,  nervous,  excretory 
(kidney),  and  reproductive  systems.  The  general  plan  of  these 
organs  may  be  learned  from  examining  an  earthworm  in  a  later 
lesson. 

Nereis  and  other  marine  segmented  worms  have  an  inter- 
esting, free-swimming  larva  in  their  development.  Figure  111 
shows  the  main  stages  in  another  segmented  worm  which 
has  such  a  stage.  The  fertilized  egg  divides  into  two,  then 
four,  eight,  sixteen,  etc.,  cells  and 
forms  a  hollow  sphere  of  cells.  Then 
one  pole  of  this  sphere  turns  inward  as 
one  might  push  in  one  hemisphere  of 
a  hollow  rubber  ball.  This  forms  two 
layers,  as  in  Fig.  Ill,  5,  the  outer 
being  ectoderm  and  the  inner  endo- 
derm.  Such  a  two-layered  embryo  is 
called  a  gastrula.  The  primitive  ali- 
mentary canal  (e  in  Fig.  Ill,  5-8) 
soon  forms  mouth  and  anus,  and  the 
embryo  grows  into  a  top-like  form. 
In  this  condition  it  begins  to  lead  an 
independent  existence  and  is  called  a 
larva.  Gradually  the  lower  end  of 
the  larva  becomes  elongated  and  seg- 
mented (Fig.  Ill,  10),  the  head  be- 
comes relatively  smaller,  and  the  larva  is  metamorphosed 
into  the  adult  worm  (11),  which  begins  to  live  in  sand.  In 
all  cases  like  this  where  the  young  animal  which  hatches 
from  the  egg  is  quite  different  from  the  adult,  the  young 
animal  is  called  a  larva,  and  the  change  to  the  adult  is 
metamorphosis. 

302.  The  Earthworm.  —  One  other  segmented  worm  de- 
serves more  than  the  brief  attention  which  our  time  will 
allow,  and  that  is  the  land  worm,  commonly  known  as  earth- 
worm or  "  fishing  worm."  It  lives  in  moist  soil  which  con- 


FIG.  112.  An  annelid 
worm  which  has  an  un- 
usual habit  of  multiply- 
ing asexually  by  dividing 
off  young  worms  in  the 
order  1  to  5.  a  is  the 
original  worm.  (After 
Milne-Edwards.) 


350  APPLIED  BIOLOGY 

tains  decaying  organic  matter,  crawls  out  at  night  to  feed, 
or  when  its  burrows  are  flooded  with  water.  It  eats  the  soil 
through  which  it  burrows,  and  its  digestive  juices  dissolve 
bacteria,  leaf -mold,  and  other  organic  matter  contained  in 
the  soil.  The  indigestible  soil  is  discharged  and  forms  the 
"  castings  "  which  are  abundant  on  the  surface  of  soil  where 
the  animals  live.  Darwin  found  places  where  he  estimated 
that  the  earthworms  brought  more  than  35,000  pounds  of 
soil  per  acre  to  the  surface  in  a  year.  This  continual  work- 
ing of  soil  by  worms  is  regarded  as  of  great  agricultural  value. 
See  Darwin's  "  Vegetable  Mold  and  Earthworms." 

General  External  Structure. —  (L)  Notice  a  living  worm  as  it  moves, 
whicn  if  doesTby  successively  elongating  and  shortening  its  body. 
Determine  anterior,  posterior,  dorsal,  and  ventral.  Compare  the 
color  on  the  dorsal  and  ventral  surfaces.  Is  the  animal  bilaterally 
symmetrical  ?  Is  there  a  head,  thorax,  and  abdomen,  as  in  the  frog  ? 
Estimate  the  number  of  segments  or  rings  in  the  body.  Locate  the 
mouth  and  arms.  Notice  the  glistening  surface  of  the  skin  due  to  a 
cuticle.  About  one  third  or  one  quarter  the  body  length  from  an- 
terior end,  there  is  a  swollen  region,  the  girdle  or  clitellum.  It  con- 
tains gland  cells  which  secrete  a  cocoon  in  which  the  eggs  are  laid 
(§  303).  The  clitellum  is  not  present  on  very  young  worms  and  on 
older  ones  only  in  spring  of  the  year,  the  egg-laying  season.  Four 
double  rows  of  small  bristles  (setae),  which  aid  the  worm  in  locomo- 
tion, may  be  located  by  pulling  a  preserved  specimen  between  a 
thumb  and  a  finger,  and  by  using  a  hand-lens.  How  many  bristles 
are  on  a  segment?  Make  dia  rams  showing  their  position. 

Living  earthworms  should  be  observed  with  regard  to  (1)  move- 
ments on  soil  and  on  hard  surfaces,  and  (2)  reactions  to  light  and 
touch. 

The  dorsal  and  ventral  blood-vessels  may  be  easily  seen  in  a 
living  animal.  Movements  of  the  dorsal  blood-vessels  can  be  easily 
seen  if  a  light-colored  worm  is  selected,  placed  on  a  moist  glass  plate, 
and  another  piece  of  moist  glass  then  placed  on  top  of  the  worm  so 
as  to  compress  the  body  slightly. 

The  Body-cavity.  —  (D)  Split  a  preserved  earthworm  lengthwise 
into  right  and  left  halves  and  note  that  the  body-cavity  (coelome) 
is  divided  by  partitions  into  cavities  corresponding  to  the  external 
division  into  segments.  Examine  the  body-wall  and  the  alimentary 


THE  WORM-LIKE  ANIMALS 


351 


canal  in  a  transverse  section.  The  body-cavity  of  a  frog  is  one  con- 
tinuous space,  while  that  of  mammals  is  divided  by  the  diaphragm 
into  the  thoracic  cavity  (containing  heart 
and  lungs)  and  the  abdominal  with  stom- 
ach, intestine,  liver,  kidneys,  and  repro- 
ductive organs. 

Internal  Organs.  —  (D)  The  teacher 
should  point  out  on  preserved  earth- 
worms, which  have  been  cut  open  along 
the  dorsal  side  and  pinned  out  on  a  narrow 
board,  the  following  systems  of  organs  :  — 

(1)  Alimentary  system  consisting  of: 
pharynx   (for  seizing  food) ;    esophagus ; 
crop  (a  reservoir  for  food) ;    gizzard  (for 
grinding   food) ;     stomach-intestine    (for 
digesting  and  absorbing  food. 

(2)  Circulatory  System.  —  A  large  blood- 
vessel on  dorsal  side  of  the  intestine;   a 
number  of  arch-like  branches  which  ex- 
tend around  the  intestine  to  a  ventral 
blood-vessel.     Through  the  skin  of  a  small 
living  earthworm  it  is  possible  to  see  nu- 
merous branches  from  these  larger  blood- 
vessels  to   all   parts   of    the   body.       In 
addition  to  blood,  the  space  (body-cavity) 
between  the  digestive  canal  and  the  body- 
wall  contains  a  fluid  which  supplements 
the  work  of  the  blood.     Blood  flows  for- 
ward in  the  dorsal  vessel  and  backward  in 
the  ventral.     The  dorsal  vessel  and  the 
arches  have  muscular  walls  which  con- 
tract   at   regular   intervals,    driving    the 
blood  forward  and  then  down  into   the 
ventral  vessel.    In  the  posterior  part  of  the 
animal  the  blood  gets  back  to  the  dorsal 
vessel,  thus  making  a  complete  circulation. 

(3)  Reproductive  Organs.  —  These  are 
large,  light  colored  organs  lying  on  right 
and  left  of  the  alimentary  canal  between 
the  tenth  and  fifteenth  segments.    The 
animal  is  hermaphroditic ;  that  is,  it  has 

both  ovaries  and  spermaries.     Very  small  tubes,  not  easily  seen,  ex- 
tend to  openings  on  the  ventral  surface  of  the  animal. 


FIG.  113.  Diagram  of  in- 
ternal organs  of  earth- 
worm as  seen  dissected 
from  dorsal  side,  c.g,  cere- 
bral ganglion  or  "  brain  "; 
ph,  pharynx;  ce,  esopha- 
gus; ao.  1,  first  of  five 
pairs  of  arches  connecting 
dorsal  and  ventral  blood- 
vessels; r,  reproductive 
organs  ;  c,  crop  in  seg- 
ment 15;  g,  gizzard; 
s.i,  stomach-intestine; 
d.v,  dorsal  blood-vessel; 
d,  septum  between  two 
segments.  (From  Sedg- 
wick  and  Wilson.) 


1 


352  APPLIED  BIOLOGY 

(4)  Excretory  Organs.  —  In  each  segment,  except  a  few  near  the 
mouth,  there  are  two  delicate  coiled  thread-like  tubes  which  connect 
with  pores  on  the  sides  of  the  body  between  the  two  rows  of  bristles. 
The  inner  ends  of  the  tubes  are  funnel-like  and  open  into  the  body- 
cavity.     Minute  blood-vessels  in  the  walls  of  these  tubes  supply 
them  with  blood,  from  which  the  cells  of  the  tubes  take  nitrogenous 
excretions.     From  time  to  time  a  small  amount  of  fluid  from  the 
body-cavity  is  allowed  to  flow  through  the  tubes  and  wash  the  ex- 
cretions to  the  exterior.     These  tubes  act,  then,  as  simple  kidneys  ; 
and  the  several  hundred  pairs  of  such  kidney-tubes  (nephridia)  in 
a  large  worm  co-operate  in  doing  the  work  such  as  the  frog's  kidneys 
(each  containing  hundreds  of  tubes)  do  for  that  animal. 

Carbon  dioxide  is  excreted  by  absorption  from  the  skin,  and  some 
also  in  the  fluid  ejected  by  the  kidney-tubes. 

(5)  Nervous  System.  —  The  main  nerve-cord  is  on  the  ventral  sur- 
face, and  is  easily  seen  in  a  specimen  from  which  the  digestive  organs 
have  been  carefully  removed.     In  each  segment  there  is  a  thickened 
portion,   known  as  a   ganglion,  and  containing   many   nerve-cells. 
Two  pairs  of  nerves  in  each  segment  run  from  the  main  nerve-cord 
to  the  various  organs  in  that  region.     Dorsal  to  the  anterior  end 
of  the  pharynx  lie  two  ganglia  (cerebral  ganglia),  sometimes  errone- 
ously called  the  "brain,"  and  from  them  two  small  nerves  extend 
around  the  esophagus  to  the  main  nerve-cord  on  the  ventral  side. 
Note  that  the  main  nervous  organs  of  the  earthworm  are  on  the 
ventral  side  of  the  digestive  canal,  while  in  the  frog  (and  all  verte- 
brates) they  are  dorsal. 

303.  Development  of  Earthworm.  —  Although  both  ova 
and  sperm-cells  are  produced  by  each  individual,  self-fer- 
tilization is  guarded  against  as  effectually  as  in  many  flowers 
we  have  studied.  In  each  worm  there  are  in  segment  10 
some  small  sacs  or  sperm-reservoirs  which  are  filled  with 
sperm-cells  from  the  spermaries  of  another  individual  when 
two  worms  pair  in  early  spring.  These  sperms  are  stored 
until  time  for  laying  eggs.  The  eggs  are  deposited  in  a  case 
or  cocoon  formed  as  follows.  A  secretion  is  poured  out  on 
the  surface  of  the  girdle  (clitellum)  of  the  worm,  this  secretion 
hardens  to  form  a  ring,  and  then  between  the  ring  and  the  body 
a  jelly-like  nutrient  substance  is  secreted.  The  ring  is  then 
worked  forward  by  contraction  of  the  worm's  body,  and, 


THE   WORM-LIKE  ANIMALS  353 

as  it  slips  past  segment  14,  at  least  one  egg-cell  is  dis- 
charged from  an  oviduct,  and  as  the  ring  passes  segment 
10,  some  stored  sperm-cells  are  ejected  from  the  sperm- 
reservoirs,  which  were  previously  filled  with  cells  from 
another  worm.  When  the  ring  slips  off  the  worm's  an- 
terior end,  its  open  ends  contract  and  it  becomes  a  closed 
egg-case.  The  egg-cell  is  fertilized  by  a  sperm-cell  and 
then  divides  into  numerous  cells,  which  form  the  body  of 
an  earthworm  embryo.  There  is  no  larva,  as  in  marine 
worms  (§301),  but  a  young  earthworm  about  one  inch  long 
is  hatched. 

304.  Animals  with  Blood.  —  All  backboneless  animals 
higher  than  and  including  the  segmented  worms,  have 
blood  or  similar  liquid  which  is  made  to  circulate,  usually 
by  a  heart.  The  functions  of  this  circulating  liquid  are  the 
same  as  in  the  frog  (§  52),  for  the  complexity  of  the  bodies  is 
such  that  there  must  be  a  circulating  medium  to  distribute 
oxygen  and  digested  food,  and  to  carry  excretions  from  dis- 
tant cells  to  the  excretory  organs. 

The  blood  of  these  animals  lower  than  the  vertebrates  is 
commonly  colorless,  resembling  lymph  of  vertebrates,  and 
containing  only  white  cells  or  corpuscles.  Red  blood-cells 
occur  only  in  backboned  animals.  Some  of  the  backboneless 
forms  have  blood  with  a  reddish  tint,  which  is  caused  by 
dissolved  haemoglobin,  the  substance  which  gives  the  red 
color  to  blood-cells  of  vertebrates.  This  dissolved  haemo- 
globin makes  it  possible  for  a  given  quantity  of  blood  to 
carry  more  oxygen  than  could  be  carried  in  colorless  blood ; 
hence  even  the  reddish  tint  of  the  blood  is  an  advantage  to 
the  animals  which  have  it.  Blood  which  does  not  have  the 
coloring  matter  can  probably  carry  about  as  much  oxygen  in 
solution  as  water  could. 

Examples  of  animals  which  have  blood  without  red  blood- 
cells  are  the  larger  worm-like  animals,  the  crabs,  lobsters, 
spiders,  insects,  clams,  oysters,  snails,  cuttle-fishes,  star- 

2A 


354  APPLIED  BIOLOGY 

fishes,  and  sea-urchins.     Reference  will  be  made  to  all  of 
these  in  the  succeeding  lessons. 

Important  Groups  of  "  Worms" 

Platyhelminthes  —  flat  worms,  tape-worms. 

Nemathelminthes  —  round  worms,  Trichina. 

Annelida  —  segmented  worms,  leech,  earthworm. 

These  are  phyla  or  primary  groups  equivalent  to  Protozoa  and 
Coelenterata.  For  other  phyla  of  worm-like  animals,  and  for  sub- 
divisions of  each  phylum,  advanced  books  of  zoology  should  be 
consulted. 


CHAPTER  XIII 


THE    ECHINODERMS 

305.  Echinoderma.  —  This  group  (a  phylum)  contains  the 
starfisftes,  sea-urchins,  crinoids  or  sea-lilies,  and  sea-cucum- 
bers, —  all  very  peculiar  animals  widely  different  from  those 
of  all  other  phyla.     All  members  of  the  phylum  are  inhabit- 
ants of  the  seas.     No  other  phylum  of  animals  is  exclusively 
marine ;  and  no  one  knows  any  good  reason  why  echinoderms 
have  not  migrated  up  rivers.     This  is  another  of  thousands 
of  well-known  biological  facts  which   appear  inexplicable. 
All  the  forms  named  above  have  peculiar  spines  on  the 
skin  (best  developed  in  sea-urchins) ;  and  the  name  Echino- 
derma means  spiny  skin.     We  shall  have  time  in  this  course 
for  only  a  brief  account  of  some  common  members  of  this 
group. 

306.  Starfish.  —  (D)    A  starfish  illustrates  the  general  plan  of 
echinoderm  structure.     The  most  common  species  of  our  Atlantic 
coast  has  a  central  disc  and  five  flexible  arms,  and  appears  radially 
symmetrical.       In 

fact,  naturalists  in 
the  early  part  of 
the  nineteenth  cen- 
tury thought  they 
were  related  to 
jelly-fishes  and 

other  coelenterates.  A  B  C 

Later  studies  have    -piG.  114.     Outlines  of  three  forms  of  starfishes,     ra, 
shown     that    star-        mouth  in  center  of  each  ;  r,  the  groove  containing 
fishes  and  all  other        the  foot-suckers.     (After  Gegenbaur.) 
echinoderms  are  bi- 
laterally symmetrical  with  reference  to  the  internal  organs.     The 
median  plane  is  marked  by  a  small  pore-plate  on  the  upper  surface 
near  the  angle  between  two  arms,  and  a  knife  passed  through  this 

355 


356  APPLIED  BIOLOGY 

plate  and  the  middle  of  the  arm  opposite  will  divide  the  body  into 
equal  halves  with  two  and  a  half  arms  each. 

On  the  lower  surface  of  the  central  disc  is  the  small  mouth,  and 
radiating  from  it  is  a  groove  on  each  arm.  Along  the  grooves  are 
the  suckers  or  "feet"  by  means  of  which  the  animal  crawls,  clings 
to  objects,  and  obtains  food. 

Internally,  there  is  a  central  stomach  and  digestive  tubes  radiating 
out  to  the  arms.  A  central  nerve-ring  has  a  branch  to  each  arm. 

There  is  a  central  ring  tube  with  a  branch  tube  to  each  arm  where 
smaller  branches  end  in  the  hollow  suckers.  This  tube  with  its 
branches  is  filled  with  water,  which  enters  at  the  small  pore-plate  on 
the  upper  side.  This  water-vascular  system  is  useful  in  respiration 
and  in  locomotion. 

307.  Other  Echinoderms.  —  It  will  be  profitable  to  ex- 
amine specimens  and  pictures  of  other  types  of  echinoderms 
for  the  sake  of  general  acquaintance.     Only  extensive  study 
would  elucidate  their  structure  and  functions.     The  forms 
collected    along    American    sea-coasts    are    "  sea-urchins " 
(hemispherical   and   covered   with   long   spines,    no   arms)  ; 
"  brittle   stars "    (starfish-like,   but  with   long  and  slender 
arms) ;    "  sand -dollars  "  or   "  sand-cakes  "    (flattened  discs 
covered   with   small   spines,    related   to   sea-urchins) ;    sea- 
cucumbers    (with   tough   and   leathery   skin,  and  group  of 
tentacles  at  one  end) ;    sea-lilies    or   crinoids    (abundant  as 
fossils  in   rocks,   and  a  few  living  species   occur   in  deep 
water). 

308.  Economic    Relations    of   Echinoderms.  —  The    star- 
fishes feed  on  oysters  and  clams,  and  are  often  so  numerous 
as  to  cause  great  loss  to  owners  of  oyster-beds.     Since  a  star- 
fish has  no  jaws  and  an  extremely  small  mouth,  it  is  evident 
that  only  by  some  unusual  method  of  feeding  could  it  eat 
an  oyster.     This  is  accomplished  as  follows :     The  starfish 
stomach  is  a  large  thin-walled  sac  which  can  be  everted 
through  the  small  mouth,  much  as  one  might  turn  a  glove- 
finger  inside  out.     A  starfish  fastens  its  suckers  on  an  oyster, 
and  then  the  stomach  covers  the  edges  of  the  oyster's  shell, 


THE  ECHINODERMS  357 

with  the  result  that  the  currents  of  water  (§  338)  are  stopped 
and  the  animal  within  the  shell  is  killed  by  suffocation.  The 
shell  then  gapes  open,  the  starfish's  stomach  pours  in  its 
digestive  secretion,  the  tissues  of  the  oyster  are  dissolved 
(digested)  while  in  its  own  shell,  then  the  digested  substances 
are  absorbed  by  the  starfish.  Finally,  the  starfish  withdraws 
its  stomach  into  its  own  body  and  leaves  the  empty  shell  of 
the  oyster. 

Against  such  a  remarkable  enemy  an  oyster  or  clam  is 
completely  helpless,  for  the  hard  shell  which  protects  against 
enemies  which  feed  like  ordinary  animals,  is  of  little  avail 
against  an  animal  peculiarly  adapted  for  suffocating  the 
oyster  and  then  digesting  its  tissues  before  taking  them  as 
food.  It  is  only  among  the  starfishes  that  there  are  animals 
able  to  evert  their  stomachs  for  the  purpose  of  surrounding 
and  digesting  food  which  is  too  large  to  be  taken  into  the 
mouth. 

Owners  of  oyster-beds  now  make  systematic  efforts  to 
destroy  starfishes.  Formerly,  the  oyster  fishermen  used  to 
break  the  arms  from  starfishes  and  throw  them  back  into  the 
sea;  but  the  discovery  that  starfishes  have  the  ability  to 
regrow  or  regenerate,  and  that  each  of  many  pieces  may 
soon  form  a  perfect  starfish,  showed  that  breaking  them  into 
pieces  simply  multiplied  them.  Now  when  starfishes  are 
caught  by  oyster-dredges,  they  are  killed  instantly  by  boiling 
water,  or  they  are  left  on  dry  land  where  they  die  quickly. 

Classes  of  Echinoderms 

Crinoidea  —  sea-lilies  or  crinoids. 
Asteroidea  —  starfishes. 
Ophiuroidea  —  brittle  stars. 
Echinoidea  —  sea-urchins. 
Holothurioidea  —  sea-cucumbers. 


CHAPTER  XIV 
THE  ARTHROPODS 

309.  Animals  with  Jointed  Legs.  —  This  is  the  meaning 
of  the  name  Arthropoda  as  applied  to  the  group  of  animals 
including  such  forms  as  crayfishes,  crabs,  spiders,  centipedes, 
and  insects.     More   than   half   of  the   existing   species   of 
animals  are  arthropods,  and  of  insects  alone  there  are  more 
species  than  in  all  the  other  groups  of  animals  taken  together. 

There  are  four  different  types  of  common  arthropods  in- 
cluded in  the  examples  named  above,  and  each  of  these  types 
represents  a  group  known  as  a  class.  The  important  classes 
are  Crustacea  (e.g.,  water-fleas,  crayfishes,  lobsters,  crabs, 
sow-bugs) ;  Arachnida  (spiders  and  scorpions) ;  Myriopoda, 
(thousand-legs  and  centipedes) ;  and  Insecta  (beetles, 
grasshoppers,  butterflies,  flies,  cockroaches,  etc.).  We  shall 
examine  examples  of  each  of  these  types  of  arthropod  animals. 

CRUSTACEANS 

310.  Structure  of  a  Crayfish,  or  Lobster.  —  (L)    Either  of  these 
animals  may  be  studied  in  order  to  gain  a  good  idea  of  a  crustacean. 
They  are  so  similar  that  a  description  written  for  study  of  one  will 
serve  as  a  guide  for  the  other,  provided  that  the  student  keeps  a 
sharp  lookout  for  little  points  of  difference.    The  crayfish  has  some 
advantages  in  being  smaller,  and  is  usually  easier  to  obtain. 

I.   External  Structure 

NOTE  :  In  studying  the  crayfish  keep  both  living  and  preserved 
(in  alcohol)  specimens  at  hand.  The  living  specimens  should  be 
kept  in  a  shallow  dish  of  pure  water  and  examined  whenever  it  is 
wished  to  learn  the  use  of  any  structure  seen  in  the  alcoholic  speci- 
mens, which  are  more  convenient  to  handle. 

358 


THE  ARTHROPODS 


359 


Notice  that  the  animal  consists  of  body  and  appendages.  Are 
the  ends  of  the  body  similar?  Observe  carefully  the  head  end 
(anterior]  and  the  hinder  end  (posterior}.  Notice  that  the  body  is 
divisible  into  an  unjointed  head  portion  (cephalothorax) ,  which 
means  head-thorax,  and  a  jointed  flexible  hinder  portion  (abdomen). 
Are  there  any  indications  of 
joints  in  the  cephalothorax? 
Is  it  flexible  ?  Notice  that  the 
animal  has  right  and  left  sides. 
Examine  several  individuals 
and  determine  whether  the 
sides  are  exactly  alike.  Ex- 
amine the  lower  (ventral)  and 
upper  (dorsal)  surfaces.  To 
what  surfaces  are  most  of  the 
appendages  attached  ?  In  how 
many  planes  could  a  knife  be 
passed  so  as  to  cut  the  animal 
into  two  similar  halves  ?  Use 
terms  anterior,  posterior,  dor- 
sal, ventral,  longitudinal  and 
transverse  in  describing  the 
position  of  planes.  Is  the  cray- 
fish bilaterally  symmetrical  ? 

Estimate  length  of  the  ani- 
mal and  also  of  the  cephalo- 
thorax and  the  abdomen 
separately.  Notice  that  the 
body  is  covered  by  a  hard, 
outer  skeleton  or  case  (exo- 
skeleton).  Does  the  same  sub- 
stance cover  the  appendages  ? 
Notice  color  of  living  animal.  FIG.  115.  Blind  crayfish  from  Mam- 
Abdomen. —  At  the  extreme  moth  Cave.  (From  Packard.) 
posterior  end  is  the  tail-fin. 

Notice  that  it  is  composed  of  a  central  flattened  structure  (telson) 
and  on  either  side  a  double  fan-like  plate.  Spread  the  tail-fin  and 
make  an  outline  drawing. 

How  many  segments  in  the  abdomen,  excluding  the  telson  ? 
Number  the  segments  of  the  abdomen,  beginning  with  the  anterior 
segment,  and  later  mark  numbers  on  drawings.  Are  the  segments 
similar  in  shape?  Sketch  abdomen  as  seen  from  the  left  side. 


360 


APPLIED  BIOLOGY 


How  many  appendages  on  each  segment  ?  Do  any  segments  lack 
appendages  ?  In  what  direction  can  the  abdomen  be  bent  ?  Can  it 
bend  laterally?  Compare  with  joints  of  appendages.  Bend  the 
abdomen  and  make  sketch  showing  the  second,  third,  and  fourth 
segments  seen  in  side  view.  Now  straighten  the  abdomen  and 
sketch  the  same  segments.  Notice  the  white  membrane  which  unites 
the  segments. 

.cephalo-thorax  abdomen 


mall  antenna  — 

large  antenna 


FIG.  116.  Crayfish  with  side  of  carapace  removed  to  expose  the  gills,  e, 
eye  ;  c,  base  of  great  claw  ;  /,  scoop  which  drives  water  out  of  gill-chamber  ; 
6,  bases  of  four  walking  legs  ;  a,  swimmerets.  (From  Morse.) 

Cephalothorax.  —  The  sides  and  dorsal  surface  are  in  this  region 
covered  by  a  hard  shield  known  as  the  carapace.  Notice  the  groove 
which  marks  the  line  between  the  head  and  thorax.  On  the  head 
portion  notice  the  beak;  the  eyes ;  the  mouth  on  the  ventral  side 
(may  be  seen  by  separating  the  appendages). 

Make  outline  drawing  of  the  animal  as  seen  in  dorsal  or  lateral 
view.  (Also  ventral  view,  if  time  permits.) 

Appendages  of  the  Abdomen.  —  What  segments  (use  numbers)  of 
the  abdomen  bear  appendages  ?  Beginning  at  the  posterior  end  of 
the  body,  carefully  use  the  fine  pointed  forceps  in  removing  the 
appendages  of  one  side  in  order,  or  examine  a  mounted  set  of  the 


THE  ARTHROPODS  361 

appendages.     Examine  all  the  appendages,  and  sketch  those  from 
the  fourth  and  sixth  segments. 

Study  a  living  crayfish  with  reference  to  the  uses  of  the  appen- 
dages of  the  abdomen. 

Gills.  —  Underneath  the  sides  of  that  part  of  the  carapace  cover- 
ing the  thorax  are  the  gills  or  organs  of  respiration.  Raise  with 
forceps  the  ventral  edge  of  the  carapace  on  one  side  and  notice  that 
a  number  of  feather-like  structures  fill  the  underlying  cavity.  They 
are  the  gills,  and  the  cavity  is  the  gill-chamber,  while  the  part  of  the 
carapace  which  covers  it  is  the  gill-cover.  With  strong  scissors,  cut 
away  the  gill-cover  on  one  side,  so  as  to  expose  the  gills.  Cover  the 
animal  with  water  and  notice  the  arrangement  of  the  gills.  Move 
some  of  the  appendages  and  notice  that  some  gills  are  attached  to 
their  bases,  while  others  are  attached  to  the  inner  wall  of  the  gill- 
chamber,  which  is  really  the  body-wall.  Note  that  the  gills  are 
outside  of  the  body-wall  and  are  therefore  external  structures. 
The  gill-cover  is  simply  a  fold  of  the  body-wall.  It  is  difficult  to 
count  gills  in  position,  and  this  will  be  done  later  after  removing  the 
appendages.  Make  a  diagram  of  the  gill-chamber  showing  arrange- 
ment of  the  gills.  In  the  extreme  anterior  end  of  the  gill-chamber 
may  be  seen  a  "paddle"  or  "scoop"  which  is  attached  to  one  of  the 
appendages  of  the  mouth.  See  Fig.  116. 

(D)  Observe  the  movements  of  this  "scoop"  in  a  living  animal 
from  which  a  small  piece  of  the  gill-cover  has  been  cut  (a  painless 
operation). 

Observe  the  direction  of  the  currents  as  shown  by  powdered 
carmine  or  indigo  placed  in  the  water  near  the  bases  of  the  posterior 
walking  legs.  In  your  diagram  of  the  gill-chamber  insert  arrows  to 
show  direction  of  the  currents. 

Appendages  of  the  Cephalothorax.  —  Observe  the  locomotion  of  a 
crayfish.  How  many  pairs  of  legs  are  used  in  walking  ? 

(D  or  L)  On  the  side  of  the  body  from  which  the  carapace  was 
cut  remove  the  appendages,  beginning  with  the  most  posterior 
walking  leg,  carefully  cutting  with  strong  scissors  the  muscular 
attachments  to  the  body.  Take  care  to  keep  the  gills  in  place  and 
the  appendages  in  order.  Examine  the  appendages  in  order  of 
removal,  comparing  with  a  mounted  series  in  order  to  make  sure 
that  your  specimens  are  complete.  Number  the  appendages, 
beginning  with  the  first  antenna.  What  appendages  bear  gills? 
Count  the  gills  which  remain  attached  to  the  body-wall.  What 
is  the  total  number? 

Do  you  find  the  appendages  of  the  cephalothorax  arranged  in 


362  APPLIED  BIOLOGY 

pairs  ?  How  many  ?  Each  pair  is  believed  to  represent  a  segment, 
as  is  evidently  the  case  in  the  abdomen ;  how  many  segments,  then, 
compose  the  cephalothorax ?  How  many  in  the  whole  body? 
Compare:  (1)  walking  legs,  and  (2)  the  mouth-parts  (appendages 
used  in  feeding). 

Study  of  Living  Crayfish.  —  Observe  the  habits  of  a  living 
crayfish  in  an  aquarium.  In  what  directions  can  it  walk?  What 
appendages  are  used  in  walking  ?  Are  there  differences  in  the  move- 
ments of  the  appendages  ?  Startle  the  animal  and  observe  its  move- 
ments. Can  it  swim  ?  In  what  direction  ?  What  appendages  are 
used  in  swimming  ?  Place  the  animal  on  its  back  and  observe  what 
appendages  are  used  in  turning  itself. 

Organs  of  Sense.  —  Do  you  find  evidence  that  the  crayfish  sees  ? 
Examine  an  eye  of  a  dead  crayfish  with  a  hand-lens,  and  later  ex- 
amine with  a  microscope  thin  sections  cut  with  a  razor.  Small 
sacs  in  the  basal  segments  of  each  antennule  have  been  called  "ears," 
but  it  has  not  been  proved  that  they  are  organs  of  hearing.  It  has 
been  shown  that  these  organs  enable  the  animal  to  keep  balanced  and 
in  the  proper  position  when  moving.  In  short,  they  perform  the 
function  of  the  semicircular  canals  in  the  human  ear,  which  "make 
us  feel"  uncomfortable  when  in  unnatural  positions  (e.g.,  head 
downward). 

With  a  bristle  or  straw  touch  a  living  crayfish  in  various  places. 
Where  is  it  most  sensitive  ?  How  does  it  use  the  antennas  ? 

There  is  some  evidence  that  the  crayfish  has  the  senses  of  taste 
iid  smell,  but  it  is  difficult  to  devise  simple  experiments  to  test  these. 

II.    Internal  Organs  of  Crayfish 

(D  or  L)  Using  strong  forceps  and  scissors,  break  and  cut 
away  the  dorsal  surface  of  the  carapace  of  a  preserved  or  recently- 
chloroformed  crayfish. 

Circulatory  System.  —  Just  under  the  center  of  the  thorax  part 
of  the  carapace  is  a  cavity  (pericardial  chamber)  in  which  lies  the 
whitish  heart,  connected  with  which  are  seven  arteries  whose  branches 
conduct  the  blood  to  all  parts  of  the  body.  The  arteries  are  best 
seen  after  injecting  some  colored  fluid  into  the  heart.  Blood 
distributed  to  all  organs  by  the  arteries  is  collected  in  irregular 
spaces  in  the  body,  from  these  it  flows  into  the  tubes  of  the  gills,  and 
thence  up  to  the  pericardial  chamber.  Valves  in  the  heart  are  ar- 
ranged so  that  blood  from  the  pericardial  chamber  can  enter  the 
heart,  but  it  cannot  go  in  the  reverse  direction. 


THE  ARTHROPODS  363 

If  a  drop  of  blood  be  taken  from  the  pericardial  chamber  of  a 
recently-killed  crayfish,  it  will  be  found  to  be  a  colorless  fluid  with 
white  cells  of  amoeboid  form.  These  cells  are  often  filled  with  gran- 
ules, which  have  been  "  eaten"  (as  an  amoeba  "  eats ")  by  the  blood- 
cells.  These  are  similar  to  the  white  blood-cells  of  vertebrates, 
which  also  have  red  cells  in  their  blood. 

Digestive  System.  —  In  front  of  the  heart  lies  the  stomach  (bluish 
color  in  fresh  specimens),  and  to  the  stomach  a  short  esophagus  leads 
from  the  mouth,  which  lies  between  the  jaws.  The  stomach  has  a 
constriction  which  marks  off  an  anterior  and  a  posterior  portion, 


FIG.  117.  Internal  organs  of  a  crayfish,  b,  "  brain"  ;  k,  kidney;  m,  mouth  ; 
s,  stomach  ;  I,  digestive  gland  or  liver  ;  r,  reproductive  gland  (spermary)  ; 
A,  heart;  i,  intestine  ;  n,  ventral  nerve-cord  ;  a,  anus.  (From  McMurrich.) 

and  by  cutting  open  the  stomach  a  peculiar  food-grinding  apparatus 
(gastric  mill)  may  be  seen  in  the  anterior  portion.  Food  is  crushed 
and  shredded  before  passing  into  the  hinder  part  of  the  stomach,  and 
passed  through  a  peculiar  strainer  before  entering  the  intestine. 

On  either  side  of  the  stomach  lies  a  large  digestive  gland  (greenish 
yellow  in  fresh  specimens),  whose  ducts  open  into  the  stomach  near 
the  intestine.  Its  primary  function  is  secretion  of  a  digestive  fluid 
which  is  able  to  prepare  foods  for  absorption.  This  fluid  is  similar 
in  function  to  the  pancreatic  secretion  in  higher  animals. 

The  intestine  extends  from  the  hinder  end  of  the  stomach,  ventral 
to  the  heart,  and  near  the  dorsal  side  of  the  abdomen,  to  the  last 
segment  where  its  opening  (anus)  is  on  the  ventral  surface.  Diges- 
tion and  absorption  of  food  which  begin  in  the  posterior  part  of  the 
stomach  continue  in  the  intestine. 

Reproductive  Organs.  —  The  male  and  female  crayfishes  are 
easily  distinguished  because  in  the  female  all  the  appendages  on  the 


364  APPLIED  BIOLOGY 

first  five  segments  of  the  abdomen  are  similar,  except  that  the  first 
pair  is  very  small;  while  those  on  the  first  and  second  abdominal 
segments  of  a  male  are  modified  into  twisted  and  pointed  appendages. 

The  ovaries  of  a  female  crayfish  lie  beneath  the  pericardial  cavity, 
and  the  two  oviducts  open  on  the  basal  segments  of  the  penultimate 
walking  legs  (third  pair).  The  spermaries  of  a  male  lie  in  the  same 
position  and  the  sperm-ducts  open  on  the  basal  segments  of  the  last 
(fourth)  walking  legs. 

At  the  egg-laying  time,  eggs  issue  from  the  oviducts  and  become 
glued  to  the  appendages  of  the  abdomen,  which  is  kept  curved  so 
that  the  tail-fin  partially  covers  the  eggs.  The  eggs  are  fertilized 
by  sperm-cells  which  before  egg-laying  were  deposited  on  the  ventral 
surface  of  the  female  near  the  openings  of  the  oviducts. 

The  young  crayfishes  when  hatched  cling  to  the  appendages  of 
the  mother  until  they  are  able  to  care  for  themselves. 

Muscular  System.  —  This  consists  of  (a)  muscles  of  the  appendages, 
and  (6)  body  muscles.  The  greater  part  of  the  abdomen  is  composed 
of  muscles  which  cause  its  movements.  Examine  an  abdomen  cut 
transversely  and  notice  the  position  of  the  intestine.  On  right 
and  left  sides  there  is  a  small  dorsal  mass  of  muscle,  and  a  larger 
ventral  mass.  Examine  these  in  the  crayfish  used  for  study  of  the 
digestive  system  and  notice  how  they  extend  into  and  are  attached 
to  the  skeleton  of  the  thorax.  The  large  ventral  muscles  bend  the 
abdomen  quickly  when  the  tail-fin  is  used  in  swimming  backwards  ; 
the  dorsal  muscles  straighten  it. 

As  an  example  of  how  some  muscles  of  the  appendages  are  at- 
tached inside  the  skeleton,  move  a  mandible  (jaw)  of  the  crayfish 
with  carapace  removed  and  note  the  movement  of  a  peculiar  conical 
muscle  which  extends  almost  vertically.  It  was  attached  to  the 
carapace  before  dissection.  Other  appendage  muscles  can  be  seen 
after  the  digestive  and  reproductive  organs  and  body-muscles  are 
removed. 

Nervous  Organs.  —  Remove  the  organs  already  studied,  taking 
care  not  to  destroy  the  nerve-cord,  which  lies  on  the  ventral  surface 
of  the  abdomen  between  the  great  body-muscles  already  mentioned. 
Note  in  each  segment  of  the  abdomen  a  thickened  portion  (ganglion) 
of  the  nerve-cord.  These  ganglia  contain  masses  of  nerve-cells, 
and  from  them  small  nerves  extend  laterally  to  various  organs.  In 
some  places  the  nerve-cord  is  seen  to  be  double. 

On  each  side  of  the  esophagus  is  a  small  nerve-cord  which  extends 
anteriorly  to  a  cerebral  ganglion  (popularly  called  "brain"),  which 
lies  in  the  median  line  below  the  beak.  Just  back  of  the  esopha- 


THE  ARTHROPODS  365 

gus  these  two  nerve-cords  unite  to  form  the  double  nerve-cord, 
which  continues  through  a  sort  of  tunnel  in  the  ventral  part  of  the 
thoracic  skeleton  to  the  part  of  the  nerve-cord  already  seen  in  the 
abdomen.  In  this  thoracic  part  of  the  nerve-cord  there  is  a  ganglion 
opposite  each  pair  of  appendages.  Each  ganglion  and  pair  of  ap- 
pendages represent  a  segment. 

Respiratory  Organs.  —  As  in  the  frog,  respiration  of  crayfish 
includes  taking  in  oxygen  and  excreting  carbon  dioxide.  There  is 
simple  absorption  of  oxygen  from  the  water  touching  the  gills  (bran- 
chiae) by  the  blood  circulating  inside  the  gills,  and  absorption  of 
carbon  dioxide  from  the  blood  by  the  water.  The  delicate  membranes 
covering  the  gills  are  well  adapted  to  such  passage  of  the  two  gases 
between  blood  and  water  separated  by  the  thin  membranes  through 
which  a  kind  of  osmosis  of  the  gases  occurs. 

Obviously,  water  from  outside  must  continually  be  coming  to  the 
gills,  otherwise  the  available  oxygen  dissolved  in  the  water  would 
soon  be  extracted  and  the  carbon  dioxide  would  become  excessive  in 
amount.  To  prevent  this,  the  water  in  the  gill-chamber  is  continually 
being  changed,  entering  around  the  legs  and  at  posterior  edge  of  the 
carapace  and  being  "scooped"  out  by  a  peculiar  appendage  at  the 
anterior  end  of  the  chamber,  near  the  mouth-parts. 

Excretory  Organs.  —  The  gills,  as  already  mentioned,  are  the 
excretory  organs  for  carbon  dioxide.  The  excretion  of  nitrogenous 
waste  is  performed  by  two  organs  which  lie  in  the  anterior  part  of 
the  body  and  just  above  the  bases  of  the  large  antennae.  On  each 
antenna  note  near  its  attachment  to  the  body  a  small  white  tubercle 
with  a  central  opening.  Into  this  opening  inject  some  colored 
fluid  (using  a  sharp-pointed  pipette),  and  note  the  organ  into  which 
the  fluid  goes.  This  is  the  antennary  gland  (a  simple  kidney). 
Notice  that  a  similar  organ  lies  above  the  antenna  on  the  other  side 
of  the  body,  i.e.,  there  is  a  pair  of  these  glands  bilaterally  arranged. 
Owing  to  their  color  in  fresh  specimens,  they  are  sometines  called 
"green  glands."  Blood  circulating  in  the  tissues  of  these  glands 
gives  out  some  of  the  nitrogenous  wastes  brought  from  the  other 
organs  of  the  body,  just  as  blood  in  the  kidneys  of  the  frog  gives  off, 
through  the  cells,  such  wastes  into  the  tubules  which  lead  into  the 
larger  ducts  of  the  kidneys. 

311.  Physiology  of  the  Crayfish.  —  The  crayfish  has  organs 
much  more  highly  differentiated  than  those  of  the  earth- 
worm, but  less  than  those  of  the  frog.  In  studying  the  struc- 
ture of  the  animal,  we  have  found  organs  of  the  following 


366  APPLIED  BIOLOGY 

systems :  supporting  (the  external  skeleton) ;  muscular 
(muscles  of  the  body  and  appendages);  digestive  (mouth, 
esophagus,  stomach,  intestine,  digestive  gland) ;  circula- 
tion (heart,  arteries,  venous  blood-spaces) ;  respiration 
(gills) ;  excretion  (gills  for  carbon  dioxide,  and  antennary 
glands  for  nitrogenous  wastes,  and  probably  some  excretions 
in  the  indigestible  materials  discharged  occasionally  from  the 
intestine) ;  nervous  (cerebral  ganglia  above  the  esophagus, 
nerve-cords  from  cerebral  ganglia  to  the  ventral  nerve-cord, 
a  double  nerve-cord  in  ventral  part  of  the  body,  and  nerves 
extending  to  various  organs  of  the  body) ;  and  the  reproduc- 
tive organs  (ovaries  and  oviducts  in  female,  spermaries  and 
sperm-ducts  in  male). 

If  we  compare  with  the  frog,  we  find  that  the  same  general 
functions  are  represented  in  the  crayfish. 

(1)  The  supporting  of  the  body  of  the  frog  is  accomplished 
by  an  internal  skeleton,  while  an  external  case  or  skeleton 
serves  the  same  purpose  in  a  crayfish. 

(2)  The  digestive  organs  in  both  animals  prepare  food  for 
absorption;    but  the  organs  for  doing  this  are  somewhat 
different  in  details  of  structure. 

(3)  Both  animals  are  so  large  that  circulating  blood  must 
carry  digested  food,  oxygen,  and  excretions ;    and  in  each  a 
heart  provides  the  motive  force  of  circulation.     The  arteries 
of  a  crayfish  remind  us  of  those  of  a  frog,  but  instead  of 
tubes  or  veins  for  returning  blood  to  the  heart,  the  crayfish 
has  irregular  spaces  between  the  various  tissues  of  its  body. 
The  heart  of  a  crayfish  is  very  much  simpler  than  that  of  a 
frog,  being  a  hollow  muscular  organ,  with  valves  arranged 
to  allow  blood  to  enter  from  the  pericardial  cavity. 

(4)  The  gills  of  the  crayfish  perform  the  same  work  as  the 
lungs  and  skin  of  the  frog.     The  blood  circulating  in  the 
capillaries  of  the  gills  absorbs  oxygen  and  gives  off  carbon 
dioxide,  just  as  does  the  blood  in  the  lungs  and  skin  of  a 
frog. 


THE  AETHEOPODS  367 

(5)  The  antennary  glands  of  crayfishes  do  the  work  of  kid- 
neys of  frogs  in  eliminating  nitrogenous  excretions. 

(6)  The  nervous  system  presides  over  coordination  (§  54), 
and  the  special  senses  in  both  frog  and  crayfish. 

(7)  The  reproductive  organs  of  both  animals  have  identical 
work;    the  ovaries  forming  ova  or  egg-cells,  the  spermaries 
producing  sperm-cells,  while  the  oviducts  and  sperm-ducts 
are  simply  tubes  for  conducting  egg-cells  and  sperm-cells 
to  the  exterior,  where  each  egg-cell  may  be  entered  and  fer- 
tilized by  a  sperm-cell. 

It  is  evident  from  above  account  that  in  the  bodies  of  a 
frog  and  a  crayfish  the  same  work  is  necessary  to  the  mainte- 
nance of  life-activities,  and  that  there  is  a  system  of  organs 
for  each  function.  In  other  words,  the  crayfish  and  the  frog 
both  have  a  high  degree  of  physiological  division  of  labor 
(§  269). 

However,  there  is  one  important  difference;  namely, 
that  the  organs  of  the  frog  are  much  more  complex  and 
more  highly  specialized.  For  example,  the  frog's  diges- 
tive system  consists  of  mouth-cavity,  esophagus,  stomach, 
small  intestine,  large  intestine,  pancreas,  and  liver;  while 
that  of  the  crayfish  consists  of  mouth,  esophagus,  stomach, 
digestive  glands,  and  intestine.  For  another,  example  of 
the  frog's  greater  complexity,  we  might  compare  its  nervous 
system  with  that  of  the  crayfish. 

However,  the  crayfish's  organs  are  complex  enough  for  the 
needs  of  its  sluggish  life.  With  the  more  highly  developed 
muscular  and  nervous  activities  of  the  highest  backboned 
animals  is  associated  more  specialization  and  complexity  of 
all  organs  which  are  essential  for  the  individual  life.  The 
great  significance  of  this  fact  is  impressed  upon  us  if  we 
stop  to  consider  that  man  (the  highest  organism)  is  espe- 
cially distinguished  by  the  complexity  of  his  nervous  organs 
along  with  remarkably  perfect  muscular  functions ;  and  that 
to  this  development  o?  the  nervous  and  muscular  systems 


3l>8  APPLIED  BIOLOGY 

we  owe  all  that  has  placed  man  so  far  above  ordinary  animal 
life.  But  such  high  development  of  the  nervous  aiul  mus- 
cular organs,  whose  activities  make  human  life  worth  living, 
would  have  been  impossible  without  parallel  increase  in  the 
complexity  of  those  organs  (digestive,  respiratory,  circula- 
tory, and  excretory)  upon  which  the  muscular  and  nervous 
organs  oVpoiul. 

312.  Molting  of  Crayfish.  —  It  is  evident  that  an  animal 
inclosed  in  a  hard  external  skeleton  like  that  of  the  crayfish 
cannot  grow  rapidly  while  surrounded  by  such  a  coat-of-mail. 
This  difficulty  is  overcome  by  periodical  shedding  of  the 
skeleton,  followed  by  rapid  increase  in  size  for  a  short  time 
while  the  skin  remains  soft  and  extensible.  The  shedding 
or  molting  occurs  several  times  in  the  first  year  of  a  cray- 
fish's life  when  it  grows  rapidly,  and  less  frequently  there- 
after. The  process  of  molting  is  in  essentials  as  follows: 
The  membrane  on  the  dorsal  side  at  the  joint  between  the 
carapace  andlihe  skeleton  of  the  abdomen  breaks,  and  through 
the  opening  thus  made  in  the  skeleton  the  animal  works 
out  the  head-thorax  and  its  appendages  from  the  carapace 
and  withdraws  the  abdomen  from  its  skeleton.  As  the  ani- 
mal emerges  from  its  old  skeleton,  it  is  seen  to  be  covered  with 
a  soft  new  skeleton;  and  within  a  short  time  it  expands 
enormously,  largely  because  of  the  absorption  of  water. 
Gradually  lime  salts  become  deposited  and  cause  the  harden- 
ing of  the  new  skeleton.  For  a  long  time  new  internal  tissues 
may  be  formed,  this  new  growth  displacing  some  of  the  water 
which  caused  the  animal  to  swell  suddenly  when  released 
from  its  old  skeleton.  It  is  evident  that  the  sudden  absorp- 
tion of  water  when  molting  makes  the  skeleton  large  enough 
to  allow  for  growth  (forming  new  cells)  for  a  long  time,  per- 
haps for  a  year.  Owing  to  the  very  rapid  growth  in  the  first. 
year,  a  lobster  five  inches  long  has  probably  molted  twenty 
times,  while  a  ten-inch  lobster  has  molted  twenty-five  timrs. 

Such  a  molting  process  occurs  in  all  arthropods,  but  the 


THE  ARTHROPODS  369 

position  of  the  break  in  the  skeleton  through  which  the 
body  is  withdrawn  varies.  If  possible,  examine  "molts"  or 
cast-off  skeletons  of  a  crab,  king-crab,  and  an  insect  (e.g., 
grasshopper  or  cicada). 

Molting  affects  crayfishes  and  all  other  arthropods  so 
severely  that  many  die  from  exhaustion,  or  possibly  because 
blood  is  lost  from  broken  appendages.  However,  the  disad- 
vantages of  molting  appear  to  be  more  than  counterbalanced 
by  the  great  protective  value  of  the  external  skeleton. 

313.  Near  Relatives  of  the  Crayfish.  —  The  remarkably 
close  similarity  between  crayfish  and  lobster  can  only  be 


Fio.  118.  A  common  shrimp  (Palaemonetes).  Eggs  shown  attached  to  the 
swimmerets  of  the  abdomen.  Its  structure  is  very  similar  to  that  of  cray- 
fish arid  lobster.  (From  Davenport.) 

explained  on  the  assumption  that  they  are  closely  related. 
If  we  see  two  strange  people  who  look  alike,  we  assume  that 
they  belong  to  the  same  family;  and  likewise  we  believe 
that  similarity  among  animals  and  plants  indicates  relation- 
ship. 

The  crayfishes,  lobsters,  shrimps,  prawns,  crabs,  —  each 
with  numerous  species,  —  are  alike  in  the  general  plan  of 
the  body,  the  same  number  of  parts,  and  the  same  arrange- 
ment of  appendages.  Since  all  of  these  have  five  pairs  of 
large  legs,  they  are  grouped  together  in  the  order  Decapoda 
(decapods),  meaning  ten  legs. 

Now,  the  crabs  are  different  from  the  lobsters  in  that  their 
abdomens  (popularly  called  "  tails  ")  have  failed  to  grow  as 
2s 


370 


APPLIED  BIOLOGY 


rapidly  as  has  the  head-thorax  part  of  the  body  (see  Fig.  120), 
and  hence  the  greater  part  of  an  adult  crab's  body  is  head- 
thorax.  Because  the  abdomen  is  short,  the  crabs  are  often 
called  the  "  short-tailed  decapods,"  while  the  lobster  and 
others  are  called  "  long-tailed."  Such  facts  of  structure  lead 
us  to  think  that  the  various  species  of  crabs  are  closely  related 


FIG.    119.      Larva   of  FIG.  120.     Young  crab   after  metamorphosis   is 

crab,     at,  antennee  ;  complete.     Note  short  abdomen  in  comparison 

a,  abdomen;  c,  cara-  with   that   of   the   larva   in   Fig.  119.     (From 

pace.  Brooks.) 

to  each  other,  and  more  distantly  related  to  the  long-tailed 
decapods. 

(D)  A  small  permanent  collection  (in  alcohol)  of  specimens  of 
various  species  of  crayfishes,  prawns,  shrimps,  and  crabs  would  be 
valuable  for  illustrating  the  general  facts  of  structure  which  seem  to 
indicate  relationship  between  these  higher  crustaceans. 

314.  Hermit-crab.  —  One  of  the  most  interesting  cases 
of  degeneration  among  crustaceans  is  that  of  the  hermit- 
crabs,  one  species  of  which  is  shown  in  Figs.  121  and  122. 
A  young  hermit-crab  takes  possession  of  a  small  shell  (formed 
by  a  snail  which  has  died),  and  backing  into  it,  twists  the 
abdomen  so  as  to  hold  itself  firmly  in  the  shell,  which  it  drags 
about  as  it  moves  from  place  to  place.  When  growth  of  the 


THE  ARTHROPODS 


371 


FIG.  121.  Hermit-crab  in  snail  shell, 
abdomen  entirely  concealed.  (After 
Emerton.) 


crab  makes  this  shell  too 

snug  for  comfort,  the  crab 

goes  in  search  of  a  larger 

shell,    and    having  found 

one,  it  quickly  withdraws 

its  abdomen  from  the  old 

shell  and  inserts  it  in  the 

new  one,  thus  exchanging 

s1  ells. 

Since  the  abdomen  and 

*'  s    appendages    are    not 

used  as  in  ordinary  long-tailed  decapods,  degeneration  occurs. 

Compare  the  appearances  of  the  abdomen  and  its  appendages 

in  the  hermit-crab  and  crayfish  (consult  the  figures,  and  ex- 
amine specimens).  The  abdominal 
appendages  remaining  on  the  hermit- 
crab  are  those  which  help  to  hold  the 
animal  in  the  snail-shell.  Moreover, 
the  abdomen  is  covered  with  a  soft 
skin  instead  of  the  hard  exo-skeleton 
of  other  crustaceans.  The  append- 
ages of  the  head-thorax  are  not  very 
different  from  those  of  ordinary 
decapods,  and  are  used  in  locomotion 
and  feeding.  The  large  claws  are 
used  to  close  the  mouth  of  the  snail- 
shell  when  the  crab  has  withdrawn 
within,  thus  protecting  the  animal. 
By  this  peculiar  mode  of  life,  the 
hermit-crab  is  able  to  protect  itself 
against  the  attacks  of  fishes  and 

other  enemies. 

A  Case  of  Mutual  Advantage:  Com- 
—  '^-  -The  snail-shells  inhab- 
its  body.    (After  Leunis.)     ited  by  hermit-crabs  are  frequently 


FIG.  122.     Hermit-crab    re- 

moved   from  snail   shell, 


372  APPLIED  BIOLOGY 

covered  externally  by  colonies  of  hydroids  (Fig.  101).  These 
eat  the  small  particles  of  food  which  float  in  the  water  when 
the  crab  is  feeding.  The  crabs  are  believed  to  be  benefited  by 
the  presence  of  the  animals  on  its  shell  because  the  hydroids 
bear  stinging  organs,  which,  if  not  painful,  are  at  least  very 
disagreeable  to  the  delicate  mouths  of  the  fishes  and  other 
enemies  that  attempt  to  steal  away  the  food  of  the  crab. 
Such  cases  where  two  species  of  animals  are  associated  to- 
gether for  mutual  advantage  are  examples  of  commensalism, 
a  term  meaning  mess-mates. 

315.  Economic  Importance  of  Decapods.  —  The  most 
valuable  of  these  larger  Crustacea  is  the  North  American 
lobster,  which  lives  on  the  ocean  bottom  within  fifty  miles 
of  the  shore  along  our  North  Atlantic  coast,  and  the  Nor- 
way species,  which  is  supplied  to  European  markets.  The 
large  size  and  the  peculiar  qualities  of  lobster  flesh  have  led 
to  such  a  market  demand  that  there  has  been  excessive  catch- 
ing in  recent  years.  The  market  value  of  the  lobsters  caught 
each  year  is  estimated  to  be  many  millions  of  dollars.  In 
recent  years  it  has  become  evident  that  steps  must  be  taken 
to  prevent  extinction  of  the  American  lobster.  Accordingly, 
the  United  States  Bureau  of  Fisheries  has  engaged  exten- 
sively in  hatching  lobster  eggs,  and  several  states  are  enforc- 
ing laws  which  prohibit  taking  short  lobsters  (under  six 
inches  long  in  some  states,  nine  in  others)  and  females  with 
eggs  attached  to  the  abdominal  appendages.  The  advan- 
tage in  hatching  lobster  eggs  artificially  is  due  to  the  fact 
that  fishes  and  other  enemies  usually  destroy  a  very  large 
number  of  eggs  or  young  larvae,  but  in  the  hatching  troughs 
they  can  be  protected  until  several  days  old  and  better  able 
to  care  for  themselves.  The  eggs  are  collected  by  brushing 
them  from  the  abdominal  appendages  of  the  females.  A 
female  a  foot  long  may  have  over  ten  thousand  eggs,  and 
giant  specimens  over  eighteen  inches  long  have  been  found 
with  more  than  150,000  eggs. 


THE  ARTHROPODS 


373 


Next  to  the  lobster,  the  crabs  are  important  as  articles  of 
human  food.  The  blue  crab  is  the  favorite.  Spider  crabs 
and  fiddler  crabs  are  not  used  as  food.  The  "  soft-shelled  " 
crabs  of  our  markets  are  simply  individuals  which  have 
recently  shed  their  shells  (i.e.,  molted),  and  the  new  shell 
has  not  had  time  to  harden  by  deposit  of  lime  salts. 

Crayfishes  have  long  been  used  for  food  in  France  and 
elsewhere  on  the  continent  of  Europe.  More  than  thirty 
years  ago  Huxley  wrote  that  Paris  paid  over  $80,000  per  year 
for  crayfishes.  In  the  United  States  they  are  found  in 
special  markets  of  the  large  cities.  Half  a  million  are  shipped 
annually  from  the  Potomac  River.  Oregon  ships  more 
than  100,000  pounds.  The  demand  is  increasing,  and  it  will 
probably  pay  to  have  crayfish  farms  on  which  to  raise  them 
for  market.  Land  too  wet  for  agriculture  might  be  so  used. 

The  shrimps  seen  in  our  markets  usually  come  from  the 
Pacific  coast,  and  the  trade  in  them  is  worth  many  hundred 
thousands  of  dollars  annually.  Only  the  abdomens  are  com- 
monly seen  in  the  markets,  and  the  bright  red  color  is  due 
to  their  having  been  boiled. 

316.  Other  Crustaceans.  —  Besides  the  decapods,  there 
are  many  thousand  species  of  crustaceans,  from  which  we 
select  a  few  for  a  brief  study  which  will 
give  us  a  better  appreciation  of  this  type 
of  arthropods. 

The  wood-lice  or  sow-bugs,  and  the 
similar  pill-bugs,  which  live  under  stones 
and  logs,  are  easily  kept  in  a  jar  contain- 
ing large  pieces  of  bark.  Small  bits  of 
bread,  butter,  apple,  and  other  foods 
should  be  placed  in  the  jar  occasionally. 
Keep  the  bark  moist,  not  wet.  The 
most  interesting  adaptation  of  these  animals  is  for  breathing. 
The  appendages  of  the  abdomen  have  been  modified  into 
breathing  organs  and  covers  for  them.  They  are  thus  fitted 


FIG.  123.  Sow-bug  seen 
from  dorsal  side. 
(From  Morse.) 


374 


APPLIED  BIOLOGY 


for  land  or  terrestrial  life,  while  most  crustaceans  have  gills 
for  aquatic  breathing. 

Lowest  Crustaceans.— Most  fresh-water  streams  and  ponds 
contain  numerous  species  of  the  smaller  and  lower  Crustacea 
known  as  copepods  and  water-fleas.  These  are  very  im- 
portant in  the  food-supply  of  many  fishes.  As  many  forms 


FIG.  124.  Dorsal 
view  of  a  copepod. 
o,  ocellus  or  eye- 
spot  ;  a,  antenna ; 
r,  reproductive  or- 
gans ;  d,  digestive 
canal ;  e,  egg- 
masses.  (From 
Brooks.) 


FIG.  125.  A  water-flea  (Daphnia).  ant.  1, 
2,  first  and  second  antennae  ;  /,  feet ; 
ht,  heart ;  d.gl,  digestive  gland ;  br.p, 
brood-pouch  in  which  embryos  develop. 
(After  Glaus.) 


as  possible  should  be  collected  with  a  fine-meshed  net,  and 
examined  with  a  hand-lens  and  low  power  of  microscope. 
Pictures  in  zoology  books  will  help  to  identify  those  which 
may  be  found  in  pond  water. 

Barnacles.  —  The  most  remarkably  modified  of  lower  crus- 
taceans are  the  barnacles  (Fig.  126)  which  are  attached  to 
floating  sea-weeds,  timbers,  and  bottoms  of  ships,  and  the 
closely-related  rock-barnacles  or  acorn-shells  which  cover 
rocks  between  high-  and  low-tide  marks  at  the  sea-shore. 
The  barnacles  were  formerly  supposed  to  be  long-necked 


THE  ARTHROPODS 


3T5 


FIG.  126.  A  goose- 
barnacle,  x,  piece 
of  wood  to  which 
the  animal  is  at- 
tached ;  n,  neck  ; 
mouth  is  opposite 
m;  s,  bivalved 
shell ;  /,  feet  which 
move  towards  the 
mouth  (m).  (From 
Smith.) 


clams,  and  are  still  so  placed  in  the  popular 

classification  of  many  fishermen.     About 

1830  a  study  of  the  developing  barnacle 

eggs  showed  that  each  egg  forms  a  small 

triangular  larva  (nauplius)  with  three  pairs 

of  legs  (Fig.  127).     In  this  condition  very 

many  crustaceans  hatch.     This  barnacle 

larva  swims  for  a  time  and  finally  settles 

down  on  a  floating  object  (or  perhaps  a 

rock  in  case  of  the  rock-barnacles),  and 

metamorphoses  into  a  barnacle.   The  larvae 

literally  swarm  in  tropical  waters,  and  pass- 
ing ships  get  their  hulls  coated  with  myriads 

of  barnacles.     These  may  grow  to  be  sev- 
eral inches  long  during  a  voyage  of  a  few 

months ;    and  such  a  growth,  of  course,  greatly  impedes  the 

vessel. 

A  strange  thing  about  the  barnacle  is  that  the  point  of 
attachment  to  rocks  and  timbers  is  the 
head ;  and  this  led  Huxley  to  define  a 
barnacle  as  "  a  crustacean  fixed  by  its 
head  and  kicking  its  food  into  its 
mouth  with  its  legs."  This  is  literally 
true,  for  the  long  legs  projecting  from 
between  the  valves  of  the  shell  do  drive 
food  towards  the  mouth. 

Certain  barnacles  become  attached 
to  crabs,  and  develop  root-like  pro- 
cesses which  penetrate  the  tissues  of 
the  crab  and  absorb  nutriment.     Then 
the   barnacle  degenerates   until  it  is 
!TeCgTnning  nothing  but  a  sac-like  tumor  on  the 
of  the  ovary ;'  i,2,st  the  body  of  the  crab,  and  the  sac  contains 

three  pairs  of  legs  present          j       th      reproductive    Organs   of    the 
at    hatchi-ng.     (From  J 

barnacle.     Such  extreme  degeneration 


FIG.  127.  The  nauplius 
stage  of  a  barnacle,  oc, 
eye-spot ;  ov 


376  APPLIED  BIOLOG? 

caused  by  a  parasitic  mode  of  life  reminds  us  of  the  dodder 
plant,  which  loses  its  roots  and  leaves,  and  has  only  its  re- 
productive structures  (flowers)  fully  developed.  Such  cases 
suggest  that  the  reproductive  function  is  the  all-important 
one  among  animals  and  plants ;  and  it  certainly  is  true  that 
many  of  the  most  remarkable  adaptations  are  arranged  to 
secure  the  perpetuation  of  a  species. 

Some  of  the  barnacles  which  are  attached  to  floating  tim- 
bers are  familiarly  known  as  "  goose  barnacles."  The  name 
originated  in  Great  Britain  many  centuries  ago  when  it  was 
generally  believed  that  a  certain  kind  of  wild  goose  did  not 
develop  from  eggs,  as  do  ordinary  birds ;  but  that  the  bar- 
nacles along  the  shore  changed  into  small  goslings.  Hence, 
they  were  named  "  goose  barnacles."  This  interesting  myth 
was  long  ago  exploded  when  the  life-histories  of  both  the 
goose  and  the  barnacle  became  known.  Even  if  we  did  not 
now  know  how  these  two  particular  animals  develop,  we  would 
refuse  to  believe  the  barnacle-goose  story,  unless  verified  by 
critical  investigation,  because  it  is  so  well  known  that  "  like 
produces  like  "  in  the  embryonic  development  of  animals  and 
plants.  Every  one  knows  that  corn  plants  do  not  develop 
from  beans  or  other  seeds,  and  frogs  do  not  come  from  eggs 
of  fishes,  birds,  cats,  or  other  kinds  of  animals.  Individuals 
of  each  kind  of  animal  or  plant  must  come  from  egg-cells 
formed  by  one  of  the  same  kind. 

ARACHNIDS 

317.  The  Spiders  and  their  Allies  :   Arachnida.  —  Familiar 
examples  of  this  group,  which  is  one  of  the  four  leading  divi- 
sions or  classes  of  Arthropoda,  are  spiders,  scorpions,  mites, 
ticks,  and  the  king-crab.     Any  common  spider  illustrates  the 
most  characteristic  structure  of  the  animals  of  this  group. 

318.  Study  of  a  Spider.  —  (L)  Note  that  the  body  consists  of  a 
cephalo- thorax  (head- thorax),  and  a  large  abdomen;  in  this  respect 
compare  with  crayfish. 


THE  ARTHROPODS 


377 


Appendages  :  No  antennas.  Compare  the  eyes  (6  or  8)  with  those 
of  crayfish.  The  anterior  mouth-appendages  are  the  mandibles  (or 
chelicerse),  the  posterior  ones  are  maxillas  (or  pedipalpi).  Each 
maxilla  has  a  long  feeler  (palpus).  The  mandibles  are  hollow  and 
at  their  tips  are  the  openings  of  the  poison-glands.  How  many 
walking  legs  ?  Compare  number  of  legs  with  those  of  crayfish.  Are 
there  appendages  on  abdomen  ? 

In  some  species  of  spiders  transverse  bands  on  abdomen  are 
supposed  to  indicate  former  segmentation.  This  may  be  seen  in  a 
scorpion.  On  ventral  side  of  abdo- 
men  :  (1)  External  openings  of  lung- 
sacs  are,  in  common  spiders,  one  on 
either  side  of  anterior  portion  of  ab- 
domen. (2)  Between  these  are  the 
openings  of  the  reproductive  organs. 
(3)  Two  or  three  pairs  of  papillae 
(spinnerets)  are  at  posterior  end  of 
abdomen  —  note  that  these  are  seg- 
mented like  legs.  (4)  A  spiracle 
(breathing  pore)  is  just  in  front  of  the 
spinnerets  in  some  spiders.  (5)  The 
posterior  opening  (anus)  of  the  ali- 
mentary canal  lies  behind  the  spin- 
nerets. 

Make  drawings  of  spider  from  dor- 
sal and  ventral  views. 

Observe  the  habits  of  living  spiders 
in  fields  and  in  vivaria.  Keep  spiders' 
eggs  under  observation  until  hatching, 
and  then  observe  habits  of  the  young,  especially  spinning. 

How  are  spiders  distinguished  from  Crustacea  ?  In  what  respects 
are  they  similar  ? 

319.  Other  Arachnids.  —  The  scorpions  live  in  warm  coun- 
tries. They  have  a  sting  at  the  end  of  the  "  tail  "  (which  is 
part  of  the  abdomen),  and  they  use  it  for  poisoning  their  prey, 
chiefly  insects  and  spiders.  Some  of  the  large  ones,  six  to 
eight  inches  long,  can  seriously  poison  man,  and  the  sting 
of  even  the  small  ones  is  painful.  This  general  form  (see  Fig. 
129)  suggests  a  similarity  to  a  lobster ;  but  the  large  pincers 
of  the  scorpion  correspond  to  the  jaws  or  mandibles  of  crus- 


FIG.  128.  Common  spider 
(Epeira  diadema).  Note  four 
pairs  of  legs,  while  insects  have 
three.  The  abdomen  is  much 
larger  than  the  cephalothorax. 
(From  Parker  and  Haswell.) 


378  APPLIED  BIOLOGY 

taceans  and  insects.  When  the  animal  is  alive  the  "  tail  " 
is  carried  over  the  back.  The  mouth  is  very  small  and  the 
scorpion  must  suck  blood  from  its  prey.  All  the  body  back 
of  the  walking  legs  belongs  to  the  ab- 
domen. They  do  not  spin  webs,  as 
do  spiders. 

Mites  and  ticks  have  the  same  ap- 
pendages as  have  spiders.  No  seg- 
ments can  be  seen.  Many  of  them 
are  parasites  on  skin  of  domesticated 
animals  and  sometimes  on  men. 

The  daddy-long-legs,  or  harvestmen, 
FIG.  129.    Scorpion.     Pair  resemble  long-legged  spiders.     They 
""S-  •"  P^ectly  harmless,  and  suck  the 

has  five  segments  in  tail    blood  from  Small  insects. 

and  a  poison-sting  at  end        The  Ung-crob  or  horse-shoe  crab 

Openings  of  four  pairs  of  \     \     *         j      i  xi.       A  AI       A- 

lung-sacs  shown  on  ante-   (Limulus),  found  along  the  Atlantic 
rior  part  of  the  abdomen.  COast,  is  the  last  of  an  ancient  race  of 

(From  Krngsley.)  ,          A        ,,  ,.  ., 

animals.  Another  species  lives  on  the 

southern  coast  of  Asia.  The  fossil  Trilobites  were  related  to 
the  king-crabs  which  lived  in  the  past  ages.  Many  points 
in  their  structure  show  that  the  king-crabs  are  related  to 
scorpions.  The  spine  of  the  king-crab  has  no  sting. 

320.  Economic  Relations  of  the  Arachnids.  —  The  spiders, 
scorpions,  and  daddy-long-legs  prey  on  insects,  many  of  which 
are  injurious  to  useful  plants  or  in  other  ways  harmful 
from  the  human  point  of  view.  The  king-crabs  are  not 
known  to  have  any  important  harmful  relations.  Their 
flesh  is  not  inedible,  as  is  commonly  assumed  because  they  are 
related  to  spiders  and  scorpions;  and  in  fact  is  said  to  be 
used  to  some  extent  in  making  a  "  chowder  "  in  some  fishing 
villages. 

Most  important  of  the  arachnids  in  their  direct  economical 
relations  are  the  mites  and  ticks  which,  as  parasites  on  the 
skin,  irritate  and  suck  blood  and  thus  interfere  with  the  health 


THE  ARTHROPODS  379 

of  farm  animals.     They  also,  in  certain  cases,  carry  the  germs 
of  dangerous  diseases  (e.g.,  Texas  fever  of  cattle). 

The  itch-mite  which  burrows  in  human  skin  is  a  member  of 
this  group.  The  mange  of  dogs  is  caused  by  another  mite. 
The  remedy  against  all  the  mites  and  ticks  which  are  skin 
parasites  consists  in  covering  the  skin  with  soap,  ointments, 
or  solutions  containing  sulphur,  tobacco,  lime,  or  other  sub- 
stances which  are  poisonous  to  these  animals.  In  treating 
the  skin  of  sheep,  cattle,  and  other  farm  animals,  it  is  usually 
most  convenient  to  dip  the  animals  into  tanks  containing 
the  solutions.  Special  machines  for  lifting  heavy  animals 
into  and  out  of  such  a  bath  have  been  devised,  and  sheep 
in  large  numbers  are  often  driven  through  a  narrow  pathway 
which  leads  into  a  long  tank  containing  hundreds  of  gallons 
of  the  solution  through  which  the  animals  must  plunge. 

MYRIOPODS 

321.  Centipedes  and  Millipedes:  Myriopoda. — These 
are  worm-like  in  form,  but  have  the  hard  exo-skeleton  and 
jointed  appendages  which  are  characteristic  of  arthropods. 
The  segments  of  the  body  are  similar,  and  each  bears  one 
or  two  pairs  of  appendages  (one  pair  in  centipedes,  two  in 
millipedes).  They  breathe  by  means  of  much-branched  air- 
tubes  (tracheae)  which  open  by  pores  on  the  surface  and 
ramify  through  the  tissues,  thus  conducting  air  directly  to 
the  cells. 

Centipedes  have  poison  glands,  and  the  bite  of  some  large 
tropical  species  is  fatal  to  small  animals  and  causes  great 
pain  to  men.  The  bite  is  inflicted  by  the  hooked  ends  of  the 
first  feet.  The  common  house-centipede  is  harmless,  and 
destroys  insects  (see  a  special  circular  of  United  States  De- 
partment of  Agriculture).  There  is  no  reason  to  fear  the 
ordinary  centipedes  of  temperate  climates,  and  the  millipedes 
("  thousand-legs")  are  also  harmless. 


380 


APPLIED  BIOLOGY 


In  order  to  make  observations  on  the  habits  of  myriopods, 
collect  them  from  under  logs  and  stones  in  early  autumn,  and 
keep  in  closed  boxes  with  pieces  of  bark,  chips,  and  leaves 
under  which  they  can  conceal  themselves.     Keep  moist,  not 
wet.     Feed  with  flies,  earthworms,  or  bits 
of   fresh   meat   for    centipedes;    various 
vegetable  foods  for  millipedes. 

That  myriopods  are  closely  related  to 
insects  is  shown  by  many  points  of  struc- 
ture, both  external  and  internal.  But  they 
are  evidently  lower  than  insects,  for  they 
have  no  wings  and  the  body  does  not  show 
differentiation  into  head  and  thorax. 


A.  B 

FIG.  130.  Two  myrio- 
pods. A,  milliped 
with  two  pairs  of 
legs  on  each  seg- 
ment (really  two 
combined  segments) . 
B,  a  centipede. 
(From  Thomson.) 


References  :  Davenport's  "  Zoology,"  Chap- 
ter V.  Jordan  and  Heath's  "  Animal  Forms," 
pp.  111-113. 

INSECTS 


322.  Prominence  of  Insects.  —  Aside 
from  the  domesticated  animals  which  be- 
long to  the  higher  vertebrates  (birds  and 
mammals),  the  insects  are  of  more  importance  in  economic 
relations  than  are  all  other  groups  of  animals  taken  together. 
But  quite  apart  from  practical  matters,  the  insects  have  long 
keen  favorite  objects  for  study  because  many  are  beautiful, 
many  have  remarkable  adaptations  to  special  conditions  of 
life,  and  many  have  wonderful  instincts  and  nervous  activities 
which  are  surpassed  only  \>y  certain  birds  and  mammals. 

More  than  half  of  the  known  species  of  animals  are  insects. 
From  200,000  to  250,000  species  are  now  known,  and  many 
newly-discovered  species  are  named  and  described  each  year. 
Some  experts  in  entomology  (that  division  of  zoology  which 
deals  with  insects)  believe  that  there  may  be  living  now  a 
million  species  of  insects. 

It  is  a  fortunate  fact  for  students  of  insects  that  they  are  all 


THE  ARTHROPODS  381 

built  on  the  same  plan  of  structure,  and  that  a  study  of  a 
few  selected  specimens  will  enable  one  to  understand  almost 
any  other  insect  which  may  be  seen.  For  such  type  studies 
the  grasshopper  and  butterfly  will  introduce  the  chief  prin- 
ciples of  insect  structure  and  life. 

323.   Study  of  a  Grasshopper.  —  (L)  This  animal  consists  of  body 
and  appendages.     Notice  three  regions  of  the  body,  —  head;  thorax, 
with  three  pairs  of  legs ;   and  a  segmented  or  jointed  abdomen.     Is 
the  grasshopper  bilaterally 
symmetrical  ?    Notice  that 
the  body  and  appendages 
have  a  hard  covering,  the 
exo-skeleton. 

Abdomen.  —  The  tip  of 
the  abdomen  varies  in  the 
two  sexes.  In  the  female 

the    abdomen    bears    two       Fl0'.  13L     Grasshopper,     a,  antenna:    w, 
„        .    ,    ,     .  wings ;    t,  tympanum  or  ear-membrane ; 

pairs  of  pointed  structures          ^  ovipositor  .    S)  Spiracles  or  breathing 
which  are  of  use  in  deposit-          p0rea  ;  I,  base  of  leg.     (From  Hatschek.) 
ing  the  eggs,  and  together 

they  are  known  as  the  ovipositor.  The  end  of  the  abdomen  in  the  male 
is  turned  upward.  Count  the  rings  or  segments  in  the  abdomen.  On 
the  first  ring  next  to  the  thorax  there  is  on  either  side  a  shining  oval 
patch,  the  organ  of  hearing.  Along  each  side  of  the  abdomen  is 
a  groove,  and  just  above  it  is  a  row  of  pores  (spiracles  or  breathing 
pores}.  Watch  the  breathing  movements  of  the  abdomen  of  a  living 
specimen.  The  spiracles  are  connected  inside  with  a  system  of 
branched  air-tubes  (Irachece),  which  ramify  through  the  body  and 
distribute  air  directly  to  the  tissues.  Compare  this  method  of 
respiration,  which  is  characteristic  of  insects,  with  that  of  the  frog. 

Thorax.  —  Judging  from  the  pairs  of  jointed  appendages  (legs), 
how  many  segments  in  the  thorax  ?  How  many  pairs  of  wings  ? 
Are  they  attached  to  first  (anterior),  second,  or  third  segment? 

Look  for  the  breathing-pores  on  the  second  and  third  segments. 

Examine  the  two  wings  and  compare  them  as  to  form,  size, 
texture,  color,  position,  and  use.  The  veins  in  the  wings  are  hollow 
tubes  which  carry  blood  and  air. 

The  characteristic  shrill  sound  made  by  katydids  is  caused  by 
rubbing  the  upper  wing  on  the  lower  wing. 

Compare  legs  from  each  segment  of  the  body  which  bears  thorn. 


382 


APPLIED  BIOLOGY 


How  many  parts  in  a  leg  ?  Are  they  similar  ?  Observe  the  hooks 
and  pads  on  the  feet  —  these  are  used  in  clinging  when  the  animal  is 
at  rest.  Notice  how  the  legs  are  adapted  for  jumping. 

Head.  —  The  head  is  attached  to  the  thorax  by  a  soft  neck. 
Examine  the  large  eyes  with  a  hand-lens,  and  with  a  microscope  ex- 
amine a  slice  from  one  of  the  eyes.  Like  the  paired  eyes  of  the  crus- 
taceans, those  of  insects  are  compound  eyes. 
Also,  several  bead-like  simple  eyes  (ocelli)  are 
in  many  insects  situated  on  the  head  between 
the  compound  eyes  (use  hand-lens).  Exam- 
ine the  long  slender  feelers  (antennae)  with 
hand-lens.  Examine  the  mouth-parts :  upper 
lip  (labrum) ;  lower  lip  (second  maxillae,  la- 
bium) ;  first  maxillae,  and  jaws  (mandibles) 
between  the  lips ;  and  a  tongue-like  organ 
within  the  mouth. 

Make  enlarged  drawings  of  a  grasshopper : 
(1)  in  side  view  with  wings  in  closed  or  rest- 
ing position,  (2)  of  front  view  of  head,  (3)  of 
a  leg,  (4)  of  side  view  of  abdomen. 

Examine  young  grasshoppers  of  various 
sizes.  How  do  they  differ  from  the  adult  ? 

Observe  method  of  locomotion,  taking  of 
food,  kinds  of  food,  and  breathing  movements 
of  grasshoppers.  When  opportunity  offers, 
carefully  observe  them  in  the  fields  and 
record  your  observations. 

Read  the  "Molting  of  the  Grasshopper," 
Chapter  IX  in  Weed's  "Life  Histories  of 
American  Insects." 

Grasshoppers  are  called  locusts  in  the 
old  world,  but  rarely  in  America.  The  so- 
called  seventeen-year  locusts  are  really 
cicadas,  belonging  to  an  entirely  separate 
order  of  insects  (the  Hemipterans). 

324.  Study  of  Butterfly,  or  Moth.  —  (L)  Examine  a  butterfly  with 
regard  to  the  points  of  structure  already  seen  in  a  grasshopper.  The 
chief  differences  to  be  observed  are:  (1)  Form  and  size  of  wings 
and  presence  of  scales  (use  hand-lens  and  microscope).  On  some 
butterflies  and  the  related  moths  the  scales  are  hair-like.  (2)  Legs 
of  butterfly  are  adapted  for  clinging,  not  jumping.  (3)  Antennae 
are  club-shaped  on  a  butterfly,  feather-like  on  a  moth.  (4)  Moutb- 


FIG.  132.  Arrangement 
of  the  larger  tubes  of 
the  air-tube  or  tra- 
cheal  system  of  a 
cockroach.  ( After 
Miall  and  Denny.) 


THE  ARTHROPODS 


383 


Thonu?  of  Butterfly 


Spiracle 


FIG. 


133.      Butterfly  larva.      (From     Dickerson's 
"Moths  and  Butterflies,11  Ginn  &  Co.) 


parts  are  adapted  for  sucking  nectar  of  flowers.  There  are  no  jaws 
for  biting  solid  food,  but  a  long  coiled  tube  consisting  of  two  halves 
closely  applied  together  is  the  sucking  organ.  Obviously  the  butter- 
fly is,  on  the  whole, 
built  on  the  grass- 
hopper plan  of  struc- 
ture. 

Larva.  —  Most  re- 
markable about  but- 
terflies and  moths  is 
their  peculiar  devel- 
opment from  egg  to 
larva  (caterpillar), 
then  to  pupa  (which 
may  be  in  a  silky 
cocoon),  and  then 
to  perfect  insect 
(imago). 

The  larva  called 
"  tomato-worm  "  (which  develops  into  a  hawk-moth)  is  excellent  for 
study.  Note:  (1)  Head,  and  its  parts.  (2)  Thorax,  with  three 
segments,  each  having  a  pair  of  legs.  (3)  Posterior  to  these  some 
segments  have  no  legs,  and  then  come  some  seg- 
ments with  peculiar  legs  adapted  to  clinging  to 
twigs  (prop  legs).  (4)  A  curved  spine  at  pos- 
terior end.  (5)  Spiracles  or  breathing  pores 
along  the  sides  of  the  body.  (6)  Color  markings 
in  living  specimens,  if  available. 

Pupa.  —  The  pupa  formed  from  the  tomato- 
worm  is  buried  in  the  soil  and  difficult  to  find. 
Pupa  from  cocoons  of  Cecropia  moth,  or  other 
large  moths,  may  be  used.  The  cases  or  covers 
of  various  organs,  as  shown  in  Fig.  134,  may  be 
seen.  Identify  the  covers  of  the  tongue,  an- 
tennae, legs,  eyes,  and  wings.  Note  spiracles  on 
sides  of  the  body.  Examine  the  movable  seg- 
ments of  the  abdomen. 

The  fragments  to  be  found  in  a  cocoon  at 
posterior  end  of  a  pupa  are  from  the  skin  which 
was  molted  after  the  cocoon  was  spun.    The  larva  molts  several 
times  before  it  is  full  grown  and  ready  to  change  to  pupal  stage. 
Cecropia  moths  will  emerge  in  February  or  March  from  cocoons 


FIG.  134.  Pupa  of 
moth,  a,  antenna 
case  ;  w,  wing  case  ; 
tongue  and  leg 
cases  between  an- 
tennae ;  ab,  a  b  d  o  - 
men.  (From 
KinQsley.) 


384 


APPLIED  mOLOGT 


kept  in  a  schoolroom,  and  much  later  if  left  outdoors.  The  habits 
of  the  living  moths  should  be  observed.  They  will  live  but  a  short 
time,  for  their  alimentary  canal  is  too  imperfect  to  use  food.  This 
is  true  only  of  some  species  of  moths  and  butterflies. 

Larva,  pupa,  and  adult  of  common  butterflies  and  moths  can  be 
identified  by  the  figures  in  Dickerson's  "Moths  and  Butterflies," 
which  also  gives  very  readable  accounts  of  the  life-histories. 


325.  The  Reproductive  Processes  of  Insects.  —  The  eggs 
of  the  simplest  and  lowest  insects  (e.g.,  the  silver-moth,  Fig. 
136)  form  young  which  at  hatching  are  like  the  adults  except 

insize.  The  young  of 
grasshoppers  and  numer- 
ous other  species  are  at 
first  without  wings,  and 
they  go  through  a  number 
of  moltings  before  they 
become  adults.  This  con- 
dition is  called  gradual  met- 
amorphosis, or  incomplete 
metamorphosis.  The  most 
complicated  development 
is  represented  by  the  but- 
terflies, moths,  beetles, 
flies,  bees,  and  wasps.  The 

and  adult  eggs    of    these    hatch    into 

worm-like  tan*  (popularly 
called  caterpillars,  grubs, 
maggots).  These  larvae  are  voracious  feeders,  and  usually 
grow  rapidly  to  full  size.  Then  while  transforming  from  the 
larva  to  the  adult  or  imago  stage,  they  undergo  a  period  of 
quiet  in  the  pupa  stage.  In  the  butterflies  the  pupa  stage  is 
often  called  a  chrysalis.  The  larvae  of  some  insects,  like  silk- 
worm, spin  a  protective  cocoon  around  themselves  as  they  pass 
into  the  pupa  stage.  After  a  period  of  quiet,  during  which 
vast  internal  changes  (complete  metamorphosis)  are  taking 


FIG. 


THE  ARTHROPODS  385 

place,  the  pupal  case  bursts  and  the  perfect  insect  emerges 
in  full-grown  state. 

Insects  which  have  larva  and  pupa  stages  do  not  grow 
in  the  winged  stage.  It  is  popularly  believed  that  the  small 
flies  seen  around  houses  are  young  ones ;  but  the  truth  is  that 
they  are  adults  of  small  species. 

Many  insects  live  longer  in  the  larval  than  in  the  adult  stage. 
Some  of  the  May-flies  (also  called  day-flies  and  ephemerids) 
live  only  a  day  or  two  as  perfect  insects.  Vast  numbers 
emerge  in  the  morning,  soon  lay  their  eggs,  are  unable  to  take 
food,  and  perish  before  the  next  day.  The  seventeen-year 
cicadas  (erroneously  called  "  locusts,"  for  this  name  belongs 
to  grasshoppers)  spend  the  greater  part  of  the  seventeen  years 
as  larvae  attached  to  roots  of  trees  beneath  ground.  In  the 
seventeenth  summer  they  come  to  the  surface,  soon  pass 
through  the  pupal  stage,  and  then  emerge  as  perfect  insects. 
A  few  of  them  may  remain  for  a  number  of  weeks,  but  vast 
numbers  lay  their  eggs  in  the  twigs  of  trees  and  die  within 
a  few  days  after  becoming  adult  insects.  The  larvae  which 
hatch  from  the  eggs  deposited  in  the  twigs  fall  to  the  ground, 
burrow,  and  remain  in  the  soil  for  nearly  seventeen  years. 
A  thirteen-year  variety  occurs  in  southern  states. 

These  insects  which  have  complete  metamorphosis  are 
regarded  as  the  highest.  The  complicated  life-history  has 
many  advantages.  The  larval  or  growing  stage  is  often 
able  to  conceal  itself  and  to  reach  food  which  perfect  insects 
could  not  get;  for  example,  the  larvae  of  numerous  species  of 
beetles,  of  the  cicadas,  and  of  flies.  In  many  cases  it  is  possible 
for  the  adult  insect  to  supply  food  for  the  larvae;  for  example, 
bees  supply  food  to  the  larvae  in  the  honeycomb,  ants  feed 
and  care  for  their  larvae,  some  wasps  paralyze  other  insects 
by  stinging  and  then  place  them  as  food  for  wasp  larvae,  and 
the  eggs  of  many  parasitic  species  are  laid  in  other  insects. 
Still  another  advantage  to  some  insects  is  the  fact  that 
their  larvae  easily  withstand  the  winter  storms  (examples 
2c 


386  APPLIED  BIOLOGY 

are  many  moths  with  cocoons).  Also,  there  is  an  advantage 
in  that  the  larvae  live  under  conditions  different  from  the 
adult  insect;  for  example,  numerous  larvae  feed  on  foliage 
which  the  adults  never  frequent  except  for  laying  eggs. 
These  are  some  of  the  apparent  advantages  which  have  come 
to  insects  with  a  complicated  life-history  consisting  of  four 
stages,  —  egg,  larva,  pupa,  and  adult ;  and  it  certainly  has 
been  worth  while  for  insects  to  develop  complete  metamor- 
phosis in  place  of  such  direct  development  of  eggs  into 
miniature  adults  as  occurs  in  the  simplest  wingless  insects 
and  also  in  the  not  distantly  related  centipedes.  No  one 
knows  why  and  how  insects  have  acquired  such  complicated 
life-histories;  but  the  known  facts  have  convinced  all  en- 
tomologists that  they  all  once  had  life-histories  even  simpler 
than  the  grasshopper  now  has,  and  that  there  has  been 
developed  greater  complexity  by  introduction  of  larva  and 
pupa  which  have  certain  advantages  in  that  they  adapt 
insects  to  peculiar  conditions  of  life,  as  suggested  above. 

Parthenogenesis. — A  peculiarity  of  the  eggs  of  some  insects 
is  that  they  develop  without  fertilization.  This  is  called 
parthenogenesis.  Throughout  the  summer  months  the  in- 
dividuals of  certain  plant-lice  are  all  females,  and  their  un- 
fertilized eggs  develop  into  females.  There  may  be  many 
such  parthenogenetic  generations  in  a  summer.  In  the 
autumn  both  males  and  females  develop  from  unfertilized  eggs, 
and  fertilized  eggs  are  produced  which  survive  the  winter 
and  develop  into  females  in  the  spring.  These  begin  a  new 
series  of  parthenogenetic  generations  which  continues  until 
the  next  autumn. 

Honey-bee  drones  (males)  develop  from  unfertilized  eggs, 
and  the  workers  (undeveloped  females)  and  "  queens " 
(mature  females)  are  from  fertilized  eggs.  Thus  mating  a  black 
drone  and  a  yellow  "  queen  "  will  result  in  half-blood  or 
hybrid  workers  and  young  "  queens,"  but  yellow  drones. 

Other  cases  of  parthenogenesis  are  found  among  insects  and 


THE  ARTHROPODS 


387 


in  some  low  crustaceans.  In  some  species  of  gall-insects  and 
scale-bugs  males  have  not  been  found  ;  and  permanent  par- 
thenogenesis is  supposed  to  be  the  rule.  This  is  not  known  to 
occur  in  any  other  group  of  animals  ;  but  there  are  well-known 
cases  of  flowers  whose  egg-cells  always  develop  without 
fertilization.  Parthenogenesis  of  both  plants  and  animals  is 
interesting  because  sexual  reproduction  is  usually  provided 
for  in  all  types  of  organisms. 

326.  Classification  of  Insects.  —  The  group  Insecta  is  a 
class,  and  it  is  divided  into  about  twenty  orders.  Most  of 
these,  however,  are  not  represented  by  many  common  insects, 
and  so  will  be  omitted  from  the  following  account. 

In  classifying  insects,  as  all  other  animals,  it  is  necessary 
to  consider  all  points  of  external  and  in- 
ternal stucture  in  order  to  determine  which 
species  are  most  alike  ;  but  it  is  a  fortunate 
fact  that  it  is  often  possible  to  identify  by 
means  of  some  one  structure.  In  the  case 
of  many  insects,  the  wings  happen  to  be  the 
convenient  parts  for  general  classification  ; 
but  similar  wings  should  not  be  taken  as 
indicating  close  relationship  if  other  organs 
are  not  homologous  (i.e.,  of  corresponding 
structure). 

The  most  common  insects  in  United 
States  are  conveniently  grouped  in  the 
following  orders:- 

Aptera.  —  Name  means  without  wings. 
Example:  the  "  silver-moth  "  or  "silver- 

r 

fish  "    (Fig.  136)  and  the  "  spring-tails." 
These  are  the  oldest  insects  now  living. 

m,  .      .^.      ,  .       ,  ,1      L     • 

They  are  primitively  wingless;  that  is, 
there  is  no  evidence  that  their  ancestors  had  wings.  Fleas 
are  secondarily  wingless,  because  of  degeneracy  caused  by 
parasitism.  Entomologists  conclude  that  the  ancestors  of 


"* 

pisma.   A  modern 
Representative    of 

the  ancient  pnmi- 

tive  insects  which 
had  ******  Pai.rs  of 

Ie8s  but  no  wings. 


388  APPLIED  BIOLOGY 

fleas  had  wings  because  embryo  fleas  show  the  beginning  of 
wings.  Hence,  such  degenerate  insects  are  not  apterans, 
but  belong  in  a  higher  order. 

Orthoptera. —  Name  means  straight  wings,  referring  to  main 
veins  of  the  wings,  or  to  the  way  they  fold  together.  Ex- 
amples :  grasshoppers,  crickets,  katydids,  stick-insects, 
cockroaches,  mantis,  earwig.  Two  pairs  of  wings,  similar 
to  those  of  grasshopper.  Mouth-parts  for  biting.  At  least 
10,000  species  are  known. 

Netted-winged  Insects.  —  Old  books  recognize  an  order 
Neuroptera  for  all  insects  which  have  two  pairs  of  netted- 
veined  wings  like  the  dragon-fly  and  May-fly ;  but  study  of  their 
other  organs  and  especially  of  their  embryonic  development 
has  shown  that  dragon-flies,  May-flies,  termites  (often  called 
white  ants),  and  others  with  netted  wings  are  not  similar 
except  in  their  wings.  The  netted-veined  wings  are  found 
on  insects  now  grouped  in  five  or  six  orders.  However,  the 
beginner  cannot  do  better  than  to  use  the  popular  name 
"  netted-veined,"  and  for  further  information  consult  the 
larger  books  on  insects  for  descriptions  of  Neuroptera  and 
other  orders  of  insects  which  exhibit  this  kind  of  wings. 

Hemiptera.  —  Name  means  half  wings,  referring  to  the  fact 
that  in  some  of  these  insects  about  half  the  wing  next  to  the 
body  does  not  show  distinct  veins.  Examples  :  all  true  bugs, 
cicadas,  squash-bug,  box-elderbug,  "  stink-bug,"  chinch- 
bug,  parasitic  lice,  bed-bug,  water-bug,  cochineal  bug,  plant- 
lice,  scale-bugs.  Four  wings,  overlapping  when  folded. 
Mouth-parts  for  piercing.  Incomplete  metamorphosis. 
About  20,000  species. 

Lepidoptera.  —  Name  means  scale-wings,  referring  to  over- 
lapping scales.  Examples :  butterflies  and  moths.  Two 
pairs  of  wings,  covered  with  flat  or  hair-like  scales.  Mouth- 
parts  for  sucking.  Complete  metamorphosis.  Butterflies 
usually  fly  in  daytime  and  have  slender  antennae  with  knob 
or  club  at  end.  Moths  are  nocturnal  and  have  feather-like 


THE  ARTHROPODS  380 

antennae.  Over  50,000  species  of  Lepidoptera  are  known. 
Consult  Dickerson's  "Moths  and  Butterflies";  Comstock's 
"How  to  Know  Butterflies";  and  Holland's  "Butterfly 
Book  "  and  "  Moth  Book." 

Coleoptera.  —  Name  means  sheath  wing,  referring  to  the 
hard  anterior  wings.  Examples  :  all  beetles,  —  weevils,  fire- 
flies, June  beetles,  blister  beetles  ("  Spanish  flies  ").  Some 
so-called  bugs,  e.g.,  lady-bird  "  bug  "  and  June  "  bug,"  are 
beetles  and  not  bugs  (i.e.,  not  Hemiptera).  Front  wings  of 
beetles  are  hard  and  horny,  and  often  called  the  elytra. 
Hind  wings  are  membranous.  Complete  metamorphosis. 
It  is  estimated  that  over  100,000  species  of  beetles  have  been 
named. 

Diptera.  —  Name  means  two  wings ;  hind  wings  absent. 
Examples  :  flies,  bot-flies,  warble-flies,  mosquitoes.  Mouth- 
parts  for  sucking  (as  in  house-fly)  or  piercing  (as  in  horse-flies). 
Complete  metamorphosis.  Larvae  commonly  called  "  mag- 
gots." Small  knobs  represent  the  hind  wings.  About  40,000 
species  are  named. 

Fleas. —  Parasitic  insects,  wingless  or  with  rudimentary 
wings.  Formerly  considered  wingless  flies,  but  now  in  a 
special  order. 

Hymenoptera.  Name  means  membrane  wing,  e.g.,  a  bee's 
delicate  wing.  Examples :  ants,  bees,  wasps,  ichneumons. 
Mouth-parts  for  both  biting  and  sucking.  Complete  meta- 
morphosis. Four  wings,  hind  pair  smaller,  few  irregular 
veins.  Probably  about  30,000  species  known. 

Practice  in  Classification  of  Insects.  —  (L)  Students  should  ex- 
amine a  set  of  insects  (a  mixed  lot  preserved  in  wood  alcohol  or 
formalin  is  best) ;  and  by  comparing  with  above  descriptions,  and 
the  well-known  examples  mentioned,  assign  the  specimens  to  the 
proper  orders.  In  an  hour  of  time  one  can  learn  to  identify  as  to 
orders  the  most  common  insects.  The  names  of  genera  and  species 
of  the  very  common  insects  can  be  found  in  the  large  books,  such  as 
Comstock's  "Manual  of  Insects,"  but  rarer  specimens  can  only  be 
identified  by  specialists. 


390  APPLIED  BIOLOGY 

327.  Useful  Insects.  —  Honey-bees  and  silkworms  are  the 
only  truly  domesticated  insects.  (SeeShaler's  "  Domesticated 
Animals.")  A  few  others  are  directly  useful  for  their  prod- 
ucts, —  "  Spanish  flies  "  (used  in  medicine),  cochineal  bug 
(cultivated  on  cactus  for  the  dyes  cochineal  and  carmine 
which  their  dried  bodies  yield),  and  the  lac-insect  (which 
produces  the  valuable  shellac  used  in  varnishes).  Some 
natives  of  Africa,  Australia,  and  Mexico  eat  certain  insects. 

As  agents  in  cross-pollination  of  flowers  insects  as  a  group 
are  worth  vastly  more  than  they  destroy;  but  it  happens 
that  many  of  the  very  destructive  insects  do  not  visit  and 
pollinate  flowers.  Our  studies  of  plants  have  made  it  clear 
that  many  of  our  most  useful  plants  depend  upon  pollination 
by  insects,  e.g.,  clover,  alfalfa,  fruit  trees,  most  vegetables. 
The  Smyrna  fig  is  now  successfully  cultivated  in  California 
because  an  insect  imported  from  Algeria  pollinates  the  flowers. 

Some  insects  are  valuable  as  destroyers  of  injurious  insects. 
Numerous  insects  have  their  insect  enemies,  some  predatory 
and  some  parasitic.  As  an  example  of  predatory  insects  may 
be  mentioned  the  history  of  the  fluted  scale-bug  which  once 
threatened  to  destroy  the  orange  groves  of  California.  The 
scale  originally  came  from  Australia,  and  there  the  ento- 
mologists of  the  United  States  Department  of  Agriculture 
found  a  natural  enemy  in  a  species  of  lady-bird  beetles. 
Some  of  these  beetles  were  imported  to  California  and  in  a  few 
years  practically  exterminated  this  species  of  scale-bug. 
Specimens  of  the  beetles  sent  later  to  other  countries  have  been 
as  successful  in  ridding  orange  and  lemon  trees  of  the  de- 
structive scale-bug.  This  is  one  example  of  the  usefulness  of 
a  predatory  insect.  Many  with  similar  habits  are  constantly 
keeping  harmful  insects  in  check. 

Parasitic  insects  are  important  .checks  on  injurious  insects. 
A  large  number  of  species  of  insects  belonging  to  the  Diptera 
and  Hymenoptera  are  parasitic  during  their  larval  stage. 
The  caterpillars  of  many  moths  and  butterflies  are  frequently 


THE  ARTHROPODS  391 

parasitized  by  larvae  of  certain  flies.  The  ichneumons 
(Hymenoptera),  of  which  more  than  ten  thousand  species  are 
named,  are  famous  parasites.  Some  of  these  insects  have 
ovipositors  three  and  four  inches  long  and  are  able  to  bore 
deeply  into  trees  in  order  to  lay  their  eggs  in  the  larvse  of 
another  insect.  "  Tomato- worms  "  (larvae  of  hawk-moth)  and 
similar  moth  larvse  are  often  seen  with  their  skins  covered 
with  small  white  cocoons.  The 
parasitic  larvae  live  inside  the 
moth  larvae  and  crawl  out  to  the 
surface  of  the  skin  when  ready  to 
pupate.  The  result  in  most  cases 
is  the  death  of  the  parasitized 
caterpillars. 

The  above  are  simply  illustra-   FlG-  137-    ichneumon-fly.   One 

,.  ,  ,  ..  ,,  of    a    group    of   insects  whose 

tive    examples    chosen    from    the         eggs  are  laid,  by  means  of  long 
thousands   of    Cases    in    which  in-         ovipositors,    in    the    larvae    of 

sect  parasites  destroy  other  in- 

sects.     In  all  such  cases  where  the  insect  host  is  harmful  the 

parasite  is  beneficial. 

Finally,  it  must  be  mentioned  that  insects  are  useful  as 
foods  for  numerous  species  of  birds.  True  it  is  often  stated 
that  the  birds  are  useful  as  destroyers  of  insects ;  but  in- 
telligent people  are  beginning  to  recognize  that  many  birds 
would  be  useful  and  well  worth  supporting  for  aesthetic 
purposes  even  if  there  were  no  injurious  insects  to  be  de- 
stroyed. Aside,  then,  from  the  great  outbreaks  of  certain 
insects,  a  limited  number  of  them  are  desirable  as  food  for 
interesting  birds. 

Also,  it  should  be  mentioned  that  many  fishes  eat  large 
quantities  of  insects,  both  adults  and  larvae.  This  is  why 
imitation  insects  are  made  as  bait  for  trout  and  other  fishes. 

328.  Injurious  Insects.  —  Those  insects  which  injure  or 
destroy  useful  plants  and  animals,  or  organic  products  (e.g., 
foods)  which  are  of  value  to  man,  or  which  injure  man  him- 


392  APPLIED  BIOLOGY 

self  (as  by  infecting  with  malaria),  are  conveniently  grouped 
as  injurious  insects.  The  truth  is  that  a  very  large  pro- 
portion of  insect  species  tend  to  be  injurious,  but  usually 
do  not  attract  attention  unless  they  become  excessively 
numerous,  or  when  man  develops  a  special  interest  in  a  par- 
ticular animal  or  plant.  For  example,  probably  more  than 
eight  hundred  species  of  insects  attack  oak  trees ;  but  it  is 
rare  that  enough  appear  on  any  one  tree  to  do  any  noticeable 
damage,  and  as  long  as  there  are  plenty  of  oak  trees  no  one 
cares  how  many  kinds  of  insects  live  on  them. 

A  few  statistical  estimates  will  give  some  idea  of  the  damage 
which  a  single  species  of  insect  can  do.  The  grasshoppers 
(Rocky  Mountain  locusts)  destroyed  crops  to  the  value  of 
$200,000,000  in  Iowa,  Missouri,  Kansas,  and  Nebraska  in  four 
years,  1874-1877.  Special  pamphlets  of  the  United  States  De- 
partment of  Agriculture  record  the  enormous  damage  done  by 
chinch-bug  on  cereal  plants,  by  Hessian  fly  on  wheat,  by  scale- 
bugs  on  fruit  trees,  by  gypsy  moth  on  forest  and  fruit  trees,  by 
cotton-boll  weevil,  and  by  numerous  others  which  do  great 
but  less  damage  then  those  mentioned.  Famous  entomolo- 
gists have  estimated  that  insects  damage  farm  crops  in 
United  States  annually  to  the  extent  of  $300,000,000.  In- 
sect damage  to  valuable  forest  trees  is  on  good  authority 
estimated  at  $100,000,000,  yearly.  Add  to  these  figures  the 
enormous  loss  of  animals  through  disease  caused  directly  or 
indirectly  by  insects ;  the  destruction  of  clothing,  foods,  and 
other  useful  articles ;  the  value  of  the  working  time  and  ex- 
pense of  treatment  of  people  who  are  ill  through  disease 
caused  by  insects  (§§  329,  330) ;  and  the  total  annual  cost  of 
insect  damage  in  this  country  is  probably  more  than  the 
combined  cost  of  the  army  and  navy  and  public-school 
system.  Such  general  estimates  suggest  the  immensity  of 
the  problem  of  dealing  with  the  injurious  insects. 

And  yet  such  statistics  must  not  be  taken  as  a  declaration 
of  war  against  insects  indiscriminately,  but  they  simply  mean 


THE  ARTHROPODS  393 

that  efforts  must  be  made  to  hold  in  check  the  ones  which 
are  noticeably  harmful.  The  ordinary  insects  which  one 
meets  during  a  long  walk  in  the  country  are  not  likely  to 
appear  in  such  numbers  or  to  develop  such  new  habits  as  to 
be  of  special  economic  interest.  Hence  there  is  no  reason 
why  we  should  destroy  them.  On  the  contrary,  this  old 
world  is  of  greater  interest  because  of  their  existence.  As  an 
illustration,  katydids  and  crickets  do  eat  some  leaves  of 
grass  and  other  plants,  but  to  many  a  person  who  is  interested 
in  nature-study  a  mid-summer  night's  chorus  by  these  insects 
is  worth  far  more  than  the  trivial  damage  they  do. 

The  Control  of  Injurious  Insects. — The  investigations  by 
entomologists  in  the  past  fifty  years  have  made  great  prog- 
ress towards  controlling  injurious  insects.  Knowledge  of 
habits  and  life-histories  have  made  it  possible  to  prevent 
destruction  of  crops.  Here  are  a  few  from  hundreds  of  ex- 
amples recorded  in  the  large  works  on  economic  entomology : 
The  discovery  that  the  fruit-moth  (codling  moth)  lays  its 
eggs  in  the  calyx  of  apple  flowers  after  the  petals  have  fallen 
suggested  the  desirability  of  spraying  the  trees  with  arsenical 
poisons  before  the  Iarva3  hatch  and  burrow  into  the  fruit. 
Grasshoppers  lay  their  eggs  a  few  inches  below  the  surface 
of  the  soil,  and  hence  shallow  plowing  in  the  autumn  will  ex- 
pose the  eggs  to  the  winter  storms.  The  scale-bugs  and  plant- 
lice  live  by  sucking  sap  from  plants,  and  hence  poisons  like 
arsenicals  could  not  reach  their  stomachs,  and  the  logical 
conclusion  is  that  they  ought  to  be  sprayed  with  lime,  sul- 
phur, or  petroleum,  which  kills  by  contact.  These  cases  are 
simply  illustrations  of  the  fact  that  all  the  satisfactory  meth- 
ods of  dealing  with  injurious  insects  are  based  upon  careful 
biological  study  of  the  species  in  question. 

329.  Mosquitoes  and  Diseases.  —  Careful  investigations 
made  in  recent  years  have  proved  that  certain  insects  are 
responsible  for  transmission  of  disease  germs.  The  most 
famous  case  is  that  of  mosquitoes  of  the  genus  Anopheles 


394  APPLIED  BIOLOGY 

(Fig.  138),  which  inject  the  malarial  organism  (§  274)  into 
the  human  blood-system.  Numerous  experiments  have 
made  it  certain  that  one  of  these  mosquitoes  must  first  suck 
blood  from  a  person  who  has  malarial  parasites  in  his  blood, 
and  it  is  now  equally  certain  that  a  bite  from  such  an  infected 
mosquito  is  the  only  way  by  which  malaria  can  be  acquired. 

It  is  also  quite  certain  that  another  species  of  mosquitoes 
is  responsible  for  transmission  of  yellow  fever,  the  germ  of 
which  is  still  unknown. 

The  discovery  that  mosquitoes  are  thus  connected  with 
certain  dreaded  diseases  has  led  to  a  study  of  the  life-histories 
and  habits  of  these  insects,  with  a  view  to  reducing  the  num- 
ber of  cases  of  malaria  and  yellow  fever.  The  following 
rules  are  now  agreed  upon  by  competent  entomologists. 

(1)  Multiplication  of  mosquitoes  should  be  checked  by 
destroying  their  breeding  places;   for  example,  by  draining 
swamps,  ponds,  and  other  places  containing  stagnant  water. 
Even  a  rain-water  barrel,  open  cistern  or  tank,  buckets,  empty 
fruit-cans,  cavities  in  trees  and  stones, —  in  short,  any  place 
where  a  small  quantity  of  water  stands  for  a  few  days  may 
serve  as  a  breeding  place,  producing  thousands  of  mosquitoes. 
As  far  as  possible  all  such  places  should  be  arranged  for  per- 
manent drainage,  and  others,  such  as  cisterns  and  barrels, 
should  be  tightly  covered  or  screened  with  netting  so  as  to 
exclude  mosquitoes  which  are  about  to  lay  eggs. 

(2)  The  larval  stages  of  mosquitoes  should  be  destroyed 
when  it  is  impracticable  to  follow  the  rule  above.     Many 
streams,  lakes,  etc.,  cannot  be  drained ;   but  stocking  them 
with  fishes  will  result  in  destruction  of  most  of  the  mosquito 
larvae.     It  is  important  in  such  cases  that  the  banks  of  the 
streams  be  freed  from  rubbish  and  graded  so  that  there  will 
be  no  small  depressions  in  which  mosquitoes  may  lay  their 
eggs  safe  from  the  attacks  of  fishes. 

Oftentimes  temporary  relief,  pending  permanent  drainage, 
may  be  gained  by  spraying  crude  petroleum  on  the  surface  of 


THE  ARTHROPODS  395 

Culex  Aaopheles 


FIG.  138.  Comparison  of  malarial  mosquito  and  common  culex.  a,  . 
b,  position  of  larvae  at  surface  of  water  ;  c,  position  of  resting  adults ; 
d,  wings  ;  e,  head  appendages — a,  antenna  ;  p,  palp  ;  t,  tongue.  Compare 
lengths  of  palps  of  the  females.  (From  Jordan's  "Bacteriology"  after  other 
authors.) 


APPLIED  BIOLOGY 


stagnant  ponds.     This  will  destroy  the  larvse  when  they  come 

to  the  surface  to  breathe.     Oil  cannot  be  used  if  it  is  desired 

to  preserve  plants  and  fishes  in  the  water. 

(3)  People  should  guard  against  infec- 
tion by  avoiding  mosquito  bites,  especially 
when  in  a  region  where  malaria  is  known 
to  occur  each  year,  or  when  there  is  an 
epidemic  of  yellow  fever  in  the  southern 
states.     The  methods   of    avoiding   bites 
are  very  simple.     Houses  should  be  well 
screened,  and  isolated  mosquitoes  resting 
on  the  walls   and   ceilings  of   bed-rooms 
should  be   killed  each  evening.     Persons 
obliged  to  be  outdoors  at  night  should 
wear  thick  and  loose  clothing,  and  mos- 
quito-proof veiling  around  the  head. 

(4)  Persons  suffering  from  malaria  should 
remember  that  their  duty  to  their  fellow- 
men  demands  that  every  possible  precau- 
tion  be  taken    against   being   bitten   by 
mosquitoes.     A  drop  of  blood  from  a  ma- 

,  respira-  larious  patient  may  infect  a  mosquito  so 

that  later  H  may  inJect  the  malarial  or§an- 

of  abdomen;  g,  isms  into  healthy  people.  In  cases  of 
gills  attached  to  yenow  fever  it  is  now  the  rule  to  quaran- 

last   segment.     B,    J  .  , 

pupa ;  o,  eye ;  be-  tine  the  patient  in  a  screened  room,  and 
tween  the  eye  and  then  make  sure  that  no  infected  mosqui- 

abdomen   are    the    .  .—,,  .  i        i  i_        i      • 

cases  containing  toes  escape.  This  can  be  done  by  closing 
antenna,  legs,  and  the  room  tightly  after  removal  of  the  pa- 
ro^tube^Tpad-"  tient  and  burning  sulphur  which  will  kill 
dies  at  end  of  ab-  any  mosquitoes  concealed  in  the  room. 

Especially  should  the  above  precautions 
be  taken  against  malaria  when  Anopheles 
mosquitoes  are  common.  Hence,  it  is  important  that  this 
species  be  easily  identified.  See  Fig.  138. 


B 

FIG.  139.  A,  mos- 
quito larva;  h, 
head ;  t,  thorax 
o,  eye ;  r 


domen. 
Folsom.) 


(From 


THE  ARTHROPODS  397 

330.  Flies  and  Disease.  —  Probably  more  important  than 
mosquitoes  as  carriers  of  disease  germs  are  the  common  house- 
flies.     It  is  a  well-known  fact  that  these  flies  persist  in  walking 
on  food,  and  long  before  disease  germs  were  known,  careful 
housewives  made  strenuous  efforts  at  keeping  them  from 
kitchens  and  dining-rooms.     Recent  bacteriological  studies 
have  disclosed  some  startling  facts  which  should  lead  to  a 
general   declaration   of   war   against   the   house-flies.     The 
facts  are  these :  A  fly  allowed  to  walk  across  a  sterile  gelatin 
plate  (§  255)  will  leave  in  its  tracks  many  bacteria  pre- 
viously acquired  by  walking  on  filth.     Now,  if  a  fly  walks 
on  sewage  containing  germs  of  typhoid  or  of  other  intestinal 
diseases,  or  on  sputum  from  a  tuberculosis  patient,  and  later 
walks  on  food  or  on  dishes  ready  to  be  used  for  food  or  drink- 
ing water,  it  may  leave  in  its  tracks  dangerous  bacteria,  which 
may  be  taken  into  the  body  with  the  food  or  water,  and  then 
cause  disease.     It  is  obvious  that  in  this  way  a  single  house-fly 
may  be  a  very  dangerous  animal. 

There  are  several  ways  of  combating  this  dangerous 
pest :  (1)  Manure  piles  and  similar  breeding  places  should 
be  removed.  (2)  Houses  should  be  carefully  screened  and 
fly-poisons,  traps,  etc.,  used  to  kill  the  few  that  succeed  in 
entering.  (3)  All  foods  should  be  carefully  guarded  against 
flies.  (4)  Arrangements  for  sewage  disposal  should  be  such 
that  flies  cannot  distribute  bacteria.  For  this  reason  sewers 
and  cesspools  that  discharge  into  porous  drain-tiles  below 
the  surface  of  soil  are  preferable  for  country  and  village 
homes.  (See  a  pamphlet  on  "  Sewage  Disposal,"  issued 
(free)  by  the  United  States  Department  of  Agriculture.) 

331.  Other  Insects  and  Disease.  —  Mosquitoes  and  house- 
flies  are  the  most  important  insects  connected  with  diseases 
in  America,  but  other  insects  may  likewise  affect  human 
health.     The  germ  of  the  terrible  African  sleeping  sickness 
is  injected  into  the  blood  by  the  bite  of  a  peculiar  fly.     The 
bacteria  of  bubonic  plague  is  probably  transmitted  by  fleas 


398  APPLIED  BIOLOGY 

which  infest  rats  in  whose  blood  the  germs  occur.  In  fact, 
any  biting  insect  which  gets  an  opportunity  to  bite  any 
person  or  animal  whose  blood  contains  disease  germs  may 
become  a  carrier  of  the  germs ;  and  any  insect  which  comes 
in  contact  with  germs  and  later  with  human  food  may  in- 
directly cause  disease. 

The  above  account  suggests  the  general  relations  of 
certain  insects  to  human  diseases.  For  more  facts  on  this 
very  important  subject  read  the  pamphlets  on  "  Mosquitoes  " 
and  "  Insects  and  Health  "  published  by  the  United  States 
Department  of  Agriculture. 

332.  Adaptations  of  Insects.  —  No  other  group  of  animals 
is  so  favorable  as  the  insects  for  study  of  adaptations  of 
structures  to  special  uses. 

Insect  legs  are  adapted  to  running  (cockroach),  leaping 
(grasshopper),  walking  (fly),  grasping  (mantis),  burrowing 
(mole-cricket),  clinging  (moths  and  butterflies),  carrying 
pollen  (bees),  and  to  still  other  special  uses.  However,  in 
all  these  cases  the  general  plan  of  a  leg  is  similar. 

The  mouth-parts  are  adapted  to  various  kinds  of  foods; 
biting  and  chewing  (grasshopper,  beetles),  piercing  (mos- 
quito, hemipterans,  and  some  flies),  licking  (house-fly), 
sucking  (butterflies),  and  cutting  (carpenter-bee). 

The  wings  are  commonly  well  adapted  to  flying,  but  in 
some  species  (e.g.,  fleas)  wings  would  be  worse  than  useless 
organs  and  have  become  degenerate.  The  front  wings  of 
beetles  are  thickened  to  -form  shield-like  covers  for  the  pos- 
terior wings.  In  protective  "adaptation  of  some  insects  the 
wings  are  remarkably  leaf -like  in  appearance. 

The  form  of  insect  bodies  is  strikingly  modified  in  many 
species.  Stick-insects  (Phasmidse)  are  orthopterans  in 
which  the  body  and  legs  are  stick-like  and  easily  mistaken 
for  sticks,  twigs,  straws,  etc.,  near  which  these  insects  live. 

Some  of  the  most  remarkable  adaptations  are  connected 
with  the  larval  stages.  Larvae  of  May-flies  and  others 


THE  ARTHROPODS 


399 


live  in  water  and  breathe  by  means  of  feathery  gills  on  the 
abdomen  (Fig.  140).  The  larvae  of  mosquitoes  have  special 
breathing  tubes  which  can  be  extended  above  the  surface 
of  the  water.  Larvae  of  lepidoptera  which  crawl  on  plants 
have  false  legs  for  supporting  the  posterior  end  of  the 
body;  but  those  of  flies,  beetles,  ants,  bees,  etc.,  develop 
in  situations  where  such  extra  legs  are  not 
required  and  they  have  only  the  three  pairs 
of  thoracic  legs. 

In  various  ways  protective  structures  for 
larvae  are  formed,  and  organs  are  specially 
adapted  for  such  work.  Examples  are : 
spinning  silky  cocoons  (by  moth  larvae), 
making  honeycomb  (by  adult  bees),  mak- 
ing paper  (by  adult  wasps),  making  cases 
or  tubes  from  bits  of  plants  or  gravel  (by  FIQ  14Q 
caddis  larvae),  and  rolling  leaves  to  form 
tubes  (by  many  larvae) . 

Aquatic  insects  are  especially  noteworthy 
for  their  adaptations.  In  some  species  oar- 
like  legs  are  fitted  for  locomotion  by  swim- 
ming, and  others  for  "  skating  "  on  water. 
Mosquito  larvae  propel  themselves  by  "wriggling"  of  the 
abdomen.  Some  aquatic  larvae  (e.g.,  mosquito  pupae)  are 
so  filled  with  air  that  they  float  at  the  surface  and  can  de- 
scend only  by  swimming.  A  velvety  covering  of  hairs  on 
the  body  and  legs  enables  some  insects  to  skate  on  the  surface 
film  of  water.  The  silvery  white  of  the  "backswimmers" 
is  due  to  a  layer  of  air  held  by  delicate  hairs.  Other  aquatic 
beetles  and  bugs  have  air-spaces  beneath  the  front  wings. 
Special  insect  books  describe  many  adaptations  to  aquatic  life. 

Adaptations  of  insects  with  respect  to  color  are  so  impor- 
tant that  a  separate  section  of  this  chapter  is  devoted  to  them. 

333.    Colors   of  Insects.  —  Many  insects  are   colored  in 
ways  which  are  apparently  useful  to  them.     Such  useful 


Larva  of 
May  fly,  with 
feathery  gills  along 
side  of  the  abdo- 
men adapting  to 
aquatic  larval  life. 
(From  Parker  and 
Haswell.) 


400  APPLIED  BIOLOGY 

colors  are  commonly  interpreted  as  (1)  protective,  (2)  ag- 
gressive, (3)  warning,  (4)  mimicry. 

(1)  Protective   coloration   is    the   most    common    form  of 
useful  colors  among  insects.     A  green  insect  on  a  green  leaf  is 
more  or  less  concealed  because  it  so  closely  resembles  its 
surroundings.     Many   insects   resemble   the   bark   of   trees 
on  which  they  rest,  and  some  both  in  form  and  color  resemble 
sticks  and  leaves.     In  still  other  ways  numerous  insects 
resemble  the  objects  near  which  they  habitually  live.     Such 
similarity  to  the  environment  is  believed  to  be  protective 
against  enemies.     Note  that  it  is  not  absolutely  protective, 
for  such  insects  are  often  captured  by  birds;   but  probably 
more  such  insects  escape  than  would  if  they  were  not  pro- 
tectively colored. 

(2)  Aggressive  coloration  is  the  term  applied  to  insects 
which  resemble  their  environment  so  that  they  can  lie  con- 
cealed from  their  approaching  prey.     This  is  found  in  certain 
predaceous  insects  like  the  mantis,  which  at  the  same  time 
are  protectively  concealed  from  such  enemies  as  birds. 

(3)  Warning  Coloration. — Many  insects  are  conspicuously 
colored,  and  appear  to  make  no  attempt  at  hiding  themselves. 
The  common  monarch  butterfly  is  an  example.     The  ex- 
planation is  that  such  insects  often  have  a  disagreeable  odor, 
flavor,  or  sting  which  repels  their  enemies;    and  hence  the 
conspicuous  color  is  a  danger  signal. 

It  has  been  learned  by  experiments  that  insect-eating 
monkeys  soon  learn  that  such  a  conspicuous  coloration  is 
associated  with  disagreeable  things  and  thereafter  will  not 
attempt  to  catch  such  insects.  This  suggests  that  a  species 
with  warning  colors  would  have  an  advantage  in  that  the 
comparatively  few  individuals  caught  might  teach  the  enemy 
that  such  brightly  colored  insects  are  not  good  to  eat.  It 
is  probable  that  very  many  cases  of  conspicuous  coloration 
in  insects  are  of  value  as  warning  colors. 

(4)  Mimicry.  —  This    means    the    resemblance    of    one 


THE  ARTHROPODS  401 

species  of  insect  to  another  which  has  warning  colors.  The 
American  viceroy  butterfly  is  a  mimic  of  the  monarch. 
Young  insect-eating  monkeys  will  eat  viceroy  butterflies, 
but  will  not  touch  them  if  they  have  first  tasted  some  un- 
palatable monarchs.  This  leads  to  the  view  that  the  viceroy 
species  gains  decidedly  by  resembling  and  thus  living  on  the 
bad  reputation  of  the  monarch,  which  has  warning  colors. 
In  tropical  countries  there  are  numerous  similar  cases  where 
a  conspicuous  insect  which  has  no  special  defense  against 
its  enemies  is  similar  in  appearance  to  another  species  whose 
conspicuous  colors  are  warning  signals,  advertising  boldly  to 
enemies  that  there  is  danger. 

The  word  "  mimicry "  suggests  conscious  imitation ;  but, 
of  course,  one  insect  resembles  another  because  it  happened 
to  be  developed  that  way.  We  do  not  know  how  the  first 
viceroys  came  to  resemble  monarch  butterflies,  but  probably 
a  butterfly  appeared  which  was  unlike  its  near  relatives 
and  more  like  monarchs.  This  resemblance  to  the  monarchs 
gave  the  first  viceroy  an  advantage.  Other  viceroys  de- 
veloped because  "  like  tends  to  produce  like,"  which  principle 
of  embryology  applies  especially  to  the  propagation  of  indi- 
vidual organisms  born  with  some  peculiarity.  Obviously, 
any  slight  advantage  gained  by  the  resemblance  to  the  mon- 
archs, which  are  not  so  liable  to  attack  as  are  more  edible 
insects,  would  have  tended  towards  the  continued  multi- 
plication of  viceroys.  It  is  thus  easy  to  suggest  how  viceroys 
might  have  been  preserved  and  allowed  to  multiply; 
but  it  has  not  yet  been  discovered  why  the  first  viceroy  hap- 
pened to  be  so  unlike  its  relatives. 

The  above  explanation  of  insect  colors  as  protective, 
aggressive,  warning,  and  mimicry  is  believed  to  apply  to 
many  insects.  It  should  be  understood  that  these  devices 
for  use  of  colors  are  not  absolutely  perfect  in  their  working. 
For  example,  green  grasshoppers  concealed  in  green  leaves  are 
often  discovered  by  birds ;  but  there  is  reason  to  believe 
2o 


402  APPLIED  BIOLOGY 

that  the  green  color  makes  it  harder  for  birds  to  find  the 
insects,  and  hence  the  color  gives  some  protective  advantage. 
Likewise,  warning  colors  are  not  always  efficient  danger 
signals;  and  mimicry  does  not  always  deceive  enemies. 
However,  there  is  probably  a  decided  advantage,  on  the  whole, 
to  species  of  insects  which  possess  any  of  these  forms  of  color- 
ation. 

334.  Colors  of  other  Animals.  —  Animals  other  than 
insects  may  have  useful  coloration.  Thus  fishes,  frogs, 
lizards,  and  many  other  animals  may  have  concealing  colora- 
tion which  may  work  either  protectively  or  aggressively.  The 
white  of  birds,  hares,  and  other  small  animals  on  arctic  snow- 
fields  is  believed  to  be  protective ;  while  the  white  of  polar 
bears  may  be  aggressive,  enabling  them  to  lie  in  wait  for 
approach  of  prey.  The  stripes  of  tigers  are  supposed  to  be 
aggressive  because  they  harmonize  so  well  with  the  lights  and 
shadows  among  the  reeds  of  jungles.  Likewise,  the  tawny 
color  of  the  lion  resembles  the  sands  of  the  desert. 

Some  of  the  most  remarkable  cases  are  those  of  frogs 
and  lizards,  which  can  quickly  change  colors  when  placed 
on  new  objects,  e.g.,  from  green  to  brown  when  moved  from  a 
leaf  to  bark  of  a  tree.  This  is  accomplished  by  a  peculiar 
arrangement  of  pigment-cells  in  the  skin.  Some  cells  contain 
green  pigment  and  some  have  darker  colors.  When  the 
green  cells  are  expanded  so  as  to  expose  their  maximum 
surface,  and  the  dark  cells  are  contracted,  the  animal's  skin 
appears  green.  Expanded  dark  cells  and  contracted  green 
cells  make  the  skin  brown  in  color.  Partial  expansion  and 
contraction  of  both  kinds  of  pigment-cells  give  intermediate 
shades.  The  cells  are  controlled  through  the  nervous  sys- 
tem and  the  eyes. 

In  the  case  of  those  birds  and  mammals  which  appear  to  have 
concealing  colors,  it  should  be  remembered  that  many  of 
the  enemies  have  a  keen  sense  of  smell,  and  so  colors  cannot 
always  be  useful  For  illustration,  quails  and  other  birds 


THE  AETHROPODS  403 

live  on  the  ground  are  more  or  less  protectively  colored. 
This  may  help  them  some  in  escaping  such  enemies  as  hawks, 
but  obviously  would  be  of  no  protection  against  weasels, 
skunks,  and  other  enemies  which  hunt  by  night,  guided  by 
odors  and  not  by  colors. 

Finally,  it  should  be  noted  that  some  colors  of  animals  are 
probably  not  useful.  For  examples,  we  may  mention  the 
beautiful  colors  concealed  in  shells  of  molluscs  (Chapter  XV), 
the  "  eye-spots  "  in  wings  of  some  moths  and  butterflies, 
and  the  gorgeous  color-patterns  of  many  birds  (e.g.,  pea- 
cock). For  all  of  these  and  very  many  more  we  have  as  yet 
no  satisfactory  explanations. 

335.  Instincts  of  Insects.  —  The  instincts  of  insects  have 
long  aroused  the  wonder  of  students  of  animal  life,  for 
under  the  guidance  of  inherited  instincts  many  insects 
exhibit  remarkable  behavior.  It  is  instinct  which  leads 
honey-bees  to  build  honeycomb,  care  for  the  young  in  true 
nurse  fashion,  give  a  different  kind  of  food  to  the  larvae 
destined  to  become  "  queens,"  follow  the  "  queen  "  when  she 
leaves  the  hive  at  swarming  time,  sting  intruders,  and  in 
various  other  ways  behave  almost  like  intelligent  beings. 
It  is  instinct  which  leads  parasitic  species  to  lay  their  eggs 
in  the  right  kind  of  larvae,  and  other  species  to  place  the 
eggs  on  plants  which  will  furnish  the  right  kind  of  food  for 
the  young  larvae.  It  is  instinct  which  leads  certain  wasps  to 
capture  and  paralyze  other  insects,  and  then  place  them  where 
wasp  larvae  hatching  from  eggs  can  later  eat  them.  In  short, 
thousands  of  cases  of  striking  behavior  of  insects  are  ap- 
parently due  to  instinct.  Some  of  the  actions  of  insects 
suggest  that  they  are  intelligent  and  capable  of  reasoning ; 
but  the  most  critical  studies  have  led  entomologists  to  the 
conclusion  that  insects  do  not  reason,  but  that  they  con- 
stantly act  instinctively  and  automatically.  How  they 
originally  acquired  their  instincts  is  entirely  unknown, 
but  that  the  instincts  are  transmitted  from  generation  to 


404  APPLIED  BIOLOGY 

generation  is  certain.  A  bee  does  not  have  to  learn  to  build 
honeycomb,  for  to  do  this  is  as  natural  as  eating  and  moving. 
It  is  an  instinctive  action. 

Sometimes  insects  seem  to  have  ways  of  communicating 
with  each  other.  An  ant  or  bee  may,  while  wandering 
around,  find  something  good  to  eat,  and  soon  many  others 
will  follow  the  first  one  from  the  nest.  This  is  probably 
due  to  the  odor  of  food  conveyed  to  the  nest  by  the  first 
insect.  Also,  ants  can  recognize  those  from  another  nest; 
but  this  is  due  to  peculiar  odors,  acquired  in  different  nests 
See  article  on  "  Communal  Life  of  Ants,"  by  Adele  M.  Fielde 
in  Nature-Study  Review,  December,  1905,  Vol.  I. 

Classes   of  Arthropoda 

Crustacea  —  crayfish,  crabs,  water-fleas. 

Onycophora — Peripatus,  a  rare  worm-like  arthropod   found  in 
tropical  countries. 

Myriopoda  —  centipedes,  millipedes. 
Arachnida  —  spiders,  scorpions,  mites,  ticks. 
Insecta  —  insects. 


CHAPTER  XV 


THE    SHELL-ANIMALS :    MOLLUSCA 

336  Examples  of  Mollusca.  —  We  can  best  understand 
what  is  meant  by  mollusks  by  referring  to  common  examples, 
such  as  mussels,  oysters,  clams,  snails,  sea-snails,  garden 
slugs,  cuttle-fishes,  and  nautilus.  At  first  this  appears 
to  be  an  assemblage  of  very  unlike  animals,  but  closer 
studies  show  many  points  of  similarity  and  lead  to  the  con- 
clusion that  these  animals 
are  so  closely  related  as  to 
deserve  grouping  in  one 
phylum,  Mollusca.  In 
this  group  there  are  more 
than  25,000  named 
species. 

Snails  and  clams  are 
usually  chosen  for  first 
study  of  the  molluscan 
plan  of  structure.  ; 

337.  Study  of  a  Clam.  — 
The  quahog  or  little-neck 
clam  and  the  long-neck 
clam  are  common  in 
markets  in  large  cities  and  near  sea-coasts;  while  elsewhere 
river-clams  may  be  obtained  and  kept  in  aquaria.  Speci- 
mens preserved  in  formalin  are  best  for  most  work  with 
the  fleshy  parts.  The  river-clams  are  best  for  demonstration 
of  structure,  the  quahog  is  next  best. 

405 


FIG.  141.  A  marine  clam.  All  below 
the  line  x-y  is  usually  embedded  in  the 
sand,  h,  hinge  of  the  shell ;  u,  umbo  ; 
I,  parallel  lines  of  growth  of  the  shell ; 
m,  edge  of  mantle  ;  /,  foot ;  e,  i,  exhalent 
and  inhalent  siphons,  the  arrows  in- 
dicating the  direction  of  currents  of 
water.  (From  Verrill.) 


406  APPLIED  BIOLOGY 

Shell.  —  (L)  Examine  some  bivalved  shells  ;  some  empty,  and  one 
with  the  clam  inside.    Note  the  following :  — 

(1)  Hinge,  which  unites  the  valves.     It  has  an  elastic  band  that 
pulls  the  valves  apart  when  the  muscles  relax,  as  when  the  animal 
dies.     Interlocking  ridges  and  tooth-like  structures  form  the  hinge- 
joint  in  some  species. 

(2)  The  hump  (umbo)  is  an  elevation  on  the  hinge  line  of  the 
shell.     It  is  the  oldest  part  of  the  shell. 

(3)  Concentric  lines  extend  outward  from  the  umbo  as  a  center. 
These  are  lines  of  growth,  and  represent  successive  additions  to  the 
shell. 

(4)  The  fleshy  tissue  extending  along  the  free  edge  of  the  shell 
between  the  two  valves  is  the  thickened  edges  of  the  two  folds  of 
the  mantle,  each  of  which  lines  a  valve  and  secretes  new  shell  mate- 
rial at  the  edge  as  indicated  by  the  lines  of  growth. 

(5)  On  an  empty  pair  of  valves,  note  the  two  pairs  of  scars  where 
were  attached  the  two  muscles  (named  adductors)  which  hold  the 
valves  together  during  life. 

(6)  The  inner  pearly  lining  of  the  shell  is  secreted  by  the  mantle. 
A  small  particle  of  sand  introduced  between  the  shell  and  mantle 
will  become  coated  with  pearly  substance.     Small  particles  of  pearly 
substance  may  become  the  center  of  solid  pearls.     The  middle  calca- 
reous and  outer  horny  layers  of  the  shell  are  secreted  at  the  edge  of  the 
mantle. 

(7)  (Z>)    Drop  a  piece  of  shell  into  strong  acid  to  demonstrate 
its  calcareous  nature.     The  gas  which  causes  the  effervescence  is 
carbon  dioxide.    (How  could  you  prove  it  ?)    Heat  a  shell  in  a  hot 
flame  or  stove  for  some  time  and  it  becomes  a  mass  of  lime. 

(8)  The  hinge  is  on  the  dorsal  side  of  the  animal,  the  opposite 
or  free  edge  of  the  shell  is  ventral.     The  siphon  or  "neck"  is  poste- 
rior, and  the  umbo  and  foot  are  near  the  anterior  end.     Holding  a 
shell  with  hinge  upward  and  the  posterior  end  towards  you,  the  right 
side  of  the  clam  is  at  your  right  hand. 

(9)  Currents  of  Water.     (D)   If  a  living  clam  is  available  in  an 
aquarium,  drop  (with  a  pipette)  some  very  muddy  water  near  the 
siphon  (popularly  called  "neck"),  and  observe  directions  of  currents 
(see  Fig.  141).     These  currents  supply  both  oxygen  and  minute 
organisms  as  food  in  the  ingoing  (inhalent)  current,  and  remove 
excretions  by  the  outgoing   (exhalent)  stream.     Bivalved  mollusks 
have  no  other  method  of  getting  food  and  oxygen.     Some  of  them 
burrow  in  mud,  wood,  and  even  soft  rock,  and  spend  their  lives 
connected  with  the  open  sea  only  by  means  of  the  siphon,  which 


THE  SHELL-ANIMALS 


407 


in  some  species  may  be  many  inches  long  so  as  to  enable  the  clam 
to  live  some  distance  down  in  its  burrow.  The  currents  are  due  to 
numerous  cilia  which  cover  organs  within  the  shell  (see  Oyster,  §  338). 
Organs  within  the  Shell.  —  (D  or  L)  The  fleshy  parts  of  a  clam, 
which  may  be  seen  after  removing  one  valve  of  the  shell,  are  as  fol- 
lows :  — 

(1)  The  two  great  adductor  muscles  which  hold   the  valves  to- 
gether.    One  is  near  the  anterior,  the  other  near  the  posterior  end. 

(2)  Mantle  lining 

each  valve,  and  be-  • 

tween  the  two  folds 
of  mantle  is  the  body 
of  the  clam. 

(3)  The  foot  is  a 
firm  muscular  struc- 
ture, which  may  be 
projected   out  from 
between  the  folds  of 
the  mantle  at  the  an- 
terior-ventral   edge. 
It  is  large  in  river 
clams,  and  small  in 
the  long-necked 
clams,  which  do  not 
move  much. 

(4)  Between  the 

foot  and  mantle  on  either  side  are  two  plate-like  gills.  Water  that 
enters  at  the  siphon  flows  around  these  and  through  pores  into 
cavities  in  the  gills,  and  thence  through  the  gill-chamber  above  the 
gills  and  out  by  exhalent  current.  Blood  (colorless)  flowing  in  blood- 
capillaries  in  the  walls  of  the  gills  is  able  to  exchange  oxygen  and 
carbon  dioxide  with  the  surrounding  water.  In  short,  the  gills  are 
the  organs  of  respiration. 

(5)  Above  the  foot  at  the  anterior  end  of  the  animal  is  the 
mouth,  just  below  the  anterior  adductor  muscle.   There  are  no  jaws, 
for  the  food  consists  of  small  organic  particles  brought  by  currents  of 
water. 

(6)  On  each  side  of  the  mouth  are  two  triangular  flaps  (called 
labial  palps).     These  serve  to  direct  the  currents  of  water  so  as  to 
carry  particles  of  food  to  the  mouth. 

(7)  From  the  mouth  the  alimentary  canal  (stomach  and  intestine) 
extends  down  into  the  foot,  where  it  is  much  coiled,  then  it  runs  up 


FIG.  142.  Organs  of  a  clam  exposed  by  removal  of 
left  valve  of  the  shell.  A ,  anterior  ;  P,  posterior  ; 
D,  dorsal ;  V,  ventral ;  a,  mantle  ;  g,  gills  ;  m, 
muscles  that  close  the  valves  together ;  /,  foot ; 
o,  position  of  mouth  ;  u,  umbo  of  shell  ;  I,  labial 
palp.  Arrows  show  direction  of  ingoing  and  out- 
going currents  of  water.  (From  Brooks.) 


408  APPLIED  BIOLOGY 

and  along  the  dorsal  side  of  the  animal  to  its  end  near  the  exhalent 
part  of  the  siphon.  A  digestive  gland  (greenish  in  color)  lies  near 
the  anterior  part  of  the  alimentary  canal. 

(8)  A  heart  and  a  pair  of  kidneys  lie  in  the  body  of  the  animal 
dorsal  to  the  gills.     Blood-vessels  extend  from  the  heart  through  all 
the  tissues  of  the  body. 

(9)  Ganglia  (masses  of  nerve-cells)  lie  near  the  mouth,  in  the 
foot,  and  near  the  posterior  end  ;  and  these  are  connected  by  nerve- 
cords.     Small  nerves  extend  to  various  organs  of  the  body. 

(10)  The  reproductive  glands  (ovaries  or  spermaries)  lie  above  the 
foot  in  the  body-cavity.     Their  cells  escape  into  the  cavities  (gill- 
chambers)  above  the  gills.     The  sperm-cells  are  carried  out  by  the 
exhalent  current  of  water  and  then  carried  by  inhalent  currents  into 
other  individuals  (females)  with  egg-cells   ready   for   fertilization. 

In  the  common  fresh-water  clams  the  fertilized  eggs  fall  into 
cavities  in  the  gills  and  remain  until  developed  into  bivalved  larvae. 
Such  clams  collected  in  late  summer  or  fall  may  have  the  outer 
gills  greatly  thickened  by  eggs  or  young  larvae  (sometimes  three 
million  in  one  clam).  They  finally  escape  in  the  exhalent  current. 
These  larvae  have  sharp  teeth  on  edges  of  their  shells,  and  by  these 
they  attach  to  skin  or  gills  of  fishes.  Having  attached,  the  skin  of 
the  fish  grows  over  (encysts)  the  clam  larva,  and  so  it  remains  while 
it  begins  to  metamorphose  into  the  clam  form.  Finally,  after 
several  weeks,  the  larva  drops  from  the  skin  of  the  fish,  falls  to 
bottom  of  the  stream,  pushes  the  lower  edge  of  its  shell  into  the  mud, 
and  begins  its  life  as  a  clam. 

Problem  :  Can  you  think  of  any  way  by  which  clam  larvae  which 
are  unable  to  swim  might  be  distributed  far  upstream  in  rivers? 
River  clams  probably  originated  near  the  sea.  How  have  they  been 
distributed  even  to  the  very  sources  of  some  of  the  smallest  brooks  of 
great  river-systems  like  that  of  the  Mississippi  ? 

The  larvae  of  marine  clams  have  no  such  parasitic  development  in 
fish,  but  escape  from  the  gill-chamber  as  a  ciliated  larva  able  to 
swim  freely  for  some  time  before  it  metamorphoses  into  a  clam. 
Hence,  marine  clams  may  be  widely  distributed  by  swimming  of 
their  larvae ;  also  by  tides  and  ocean  currents. 

338.  Relatives  of  the  Clams:  Lamellibranchs.  —  The  terms 
"  clam  "  and  "  mussel "  are  applied  to  a  number  of  marine  and 
fresh-water  species  of  mollusks  which  have  a  bivalved  shell. 
There  are  two  common  types  of  river  clams,  one  with  thick 
shells  and  one  with  thin  shells.  Of  each  type  there  are 


THE  SHELL-ANIMALS  409 

many  species  indicated  by  minor  differences  in  shape  and 
markings  on  the  shells. 

The  two  most  common  marine  dams  sent  to  market  in 
eastern  states  are  the  hard  shell,  "  little  neck,"  or  "  quahog  " 
clams  of  the  genus  Venus  (Fig.  141),  which  are  the  favorites 
in  New  York  markets ;  and  the  soft  shell,  "  long  neck  " 
clams  of  the  genus  Mya,  which  are  demanded  by  the  best 
trade  in  the  Boston  markets.  The  "  little  neck "  and 
"  long  neck  "  refers  to  the  length  of  the  siphons. 

The  giant  clams  of  the  Indian  Ocean  sometimes  weigh 
500  pounds  and  may  be  two  feet  long.  The  smallest  bi- 
valve is  a  little  fresh-water  species  1  cm.  long. 

Most  remarkable  of  the  allies  of  the  clams  is  the  ship-worm 
(Teredo)  which  bores  burrows  in  ships'  timbers,  piles,  etc. 
The  body  of  the  teredo  is  worm-like.  The  small  shell, 
used  for  cutting  the  burrow,  is  at  the  inner  end  of  the  burrow. 
The  siphons  project  above  the  surface  of  the  wood  whenever 
the  "  worm  "  is  not  disturbed,  and  the  ingoing  current  of 
water  carries  in  food  (small  organisms)  and  provides  for 
respiration.  The  remedy  against  these  animals  is  copper 
sheathing,  or  soaking  timbers  with  creosote  and  tar. 

The  oyster  is  peculiar  in  that  the  eggs  develop  within  a 
few  hours  after  fertilization  into  free-swimming  larvae. 
These  finally  become  attached  and  metamorphosed  into 
the  oyster  form.  The  left  valve  of  an  oyster's  shell  is  at- 
tached to  a  stone  or  to  another  oyster  shell.  In  conse- 
quence of  this  sedentary  mode  of  life  the  oyster  has  no  use  for 
a  foot  (organ  of  locomotion),  and  this  is  absent.  One  reason 
why  the  flesh  of  oysters  is  so  delicate  in  texture  is  that  there 
is  no  hard  muscular  foot;  and  the  great  development  of 
this  organ  makes  river  mussels  "  too  tough  "  to  be  palatable. 
The  mouth  of  an  oyster  is  near  the  ligament  which  hinges 
the  shells  together;  but  as  in  case  of  clams,  currents  of 
water  from  the  opposite  side  of  the  shell  carry  in  food 
particles. 


410 


APPLIED  BIOLOGY 


(D)  Locate  oyster's  mantle,  gills,  hinge  of  shell,  position  of  mouth, 
and  one  central  adductor  muscle  which  holds  the  valves  closed. 
Mount  and  examine  microscopically  a  piece  of  gill  from  an  oyster 

just  opened,  using  for 
mounting  some  of  the 
salt  water  found  inside 
the  shell,  and  observe 
the  movements  of 
cilia,  which  produce 
currents  of  water. 

Lamellibranchs. 
—  All  the  clams, 
mussels,  oysters, 
and  other  mollusks 
with  bivalved 
shells  belong  to  a 
This  name  (from  lamella, 


FIG.  143.  Ciliated  epithelial  cells,  a,  cells  with 
one  cilium  each  ;  b,  with  many  cilia;  62  is  end 
of  view  of  b 1 ;  c,  ciliated  cells  with  underlying 
cells,  the  whole  forming  a  stratified  epithelium. 
Similar  ciliated  cells  on  oyster  gills  cause  move- 
ment of  water.  (From  Hatschek.) 


class  known  as  Lamellibranchiata. 

plate ;  and  branchia,  gill)  refers  to  the  plate-like  gills  such  as 

the  clams  and  oysters  have. 

339.  Study  of  Land-Snail.  —  (L)  The  common  European  garden- 
snail  (Helix  pomatia),  sold  in  markets,  and  native  snails  found  in 
damp  woods,  may  be  used  for  study. 
When  imported  from  Europe  the  snails 
are  in  the  hibernating  condition  with 
shell  sealed  by  a  plate  of  hardened  slime. 
If  placed  on  damp  sod  or  paper  in  a 
warm  room,  the  animal  will  "come  out " 
of  its  shell  within  a  day  or  two,  and 
begin  to  move  and  feed.  Specimens 
for  study  should  be  allowed  to  crawl  on 
plates  of  glass.  Observe  the  following 
points  of  structure  and  habits  :  (1)  foot, 
and  its  movements  as  seen  through  the 
glass  ;  (2)  the  head,  with  four  tentacles ; 

(3)  the   position   of   the   spiral   shell; 

(4)  the  collar  at  the  mouth  of  the  shell ; 

(5)  the    breathing   pore    in    the  collar 

(watch  its  movements) ;  (6)  the  mouth  (best  seen  through  the  glass 
from  below) ;  (7)  the  eyes  at  ends  of  two  of  the  tentacles.  Is  the  snail 
bilaterally  symmetrical  ? 


FIG.  144.  Diagram  of  a  land- 
snail  (Helix).  s,  shell ;  a, 
anus ;  b,  breathing  pore 
opening  into  the  "lung"; 
r,  opening  of  reproductive 
organs  ;  t,  four  tentacles  ; 
e,  eyes  at  end  of  longer  ten- 
tacles. (From  Parker  and 
Haswell.) 


THE  SHELL-ANIMALS  411 

(D)  Internal  organs.  The  form  of  the  body  must  be  studied  in 
museum  specimens  prepared  by  special  methods,  and  preserved  in 
formalin.  The  breathing  pore  leads  into  the  lung-sac.  The  pos- 
terior end  of  the  digestive  canal  is  near  the  breathing  pore,  the  canal 
having  been  twisted  so  as  to  bring  its  opening  near  the  edge  of  the 
shell.  Heart,  liver,  kidney,  loop  of  intestine,  and  reproductive 
glands  make  up  most  of  the  mass  within  the  shell.  Note  that  this 
mass  of  organs  has  the  form  of  the  shell,  and  that  the  upper  whorls 
are  filled  even  when  the  snail's  foot 
is  out  of  the  shell.  The  animal  is 
hermaphroditic  (§283).  The  nerv- 
ous system  consists  of  masses  of 
nerve-cells  (ganglia),  chiefly  placed 
anteriorly  in  head  and  foot,  and  FlG-  145-  Garden  slug  (Limax). 
with  nerves  from  these  ganglia  to  '•  *fntacles :  "V  mantle  covering 
various  parts  of  the  body.  Careful 
study  of  the  internal  organs  of  a 
snail  is  usually  made  in  college  courses  of  zoology,  but  we  have 
not  time  in  this  course. 

(L)  Study  of  empty  snail-shell.  The  small  end  is  the  apex, 
the  central  axis  is  the  columella,  and  the  opening  is  the  mouth  of  the 
shell.  Notice  direction  of  the  spiral  as  compared  with  threads  on 
an  ordinary  wood-screw.  At  a  broken  edge  may  be  seen  an  inner 
pearly  layer,  a  middle  thick  layer,  and  a  thin  outer  layer.  The 
middle  and  outer  layers  are  secreted  by  the  collar,  while  the  whole 
inner  skin  (which  is  the  mantle)  secretes  pearly  lining.  Examine  a 
shell  from  which  one  side  has  been  chipped  away  with  strong  forceps 
so  as  to  expose  the  interior. 

Garden-slugs,  which  are  usually  to  be  found  in  gardens  and  green- 
houses, are  similar  to  snails,  except  that  the  shell  is  small  and  covered 
with  skin.  Obviously,  such  a  degenerate  shell  is  of  little  use  as  a 
protecting  organ. 

340.  Snails  and  their  Allies :  Gasteropods.  —  The  garden- 
snails  are  members  of  a  group  of  mollusks  known  as  gastero- 
pods  (class  Gasteropoda).  The  name  means  stomach  in  the 
foot.  Since  they  have  a  shell  consisting  of  one  piece, 
they  are  often  called  univalves  to  distinguish  them  from  the 
bi-valved  clams. 

While  the  land-snails  and  their  near  relatives,  the  slugs, 
breathe  air  by  means  of  a  simple  lung-chamber,  most  gastero- 


412  APPLIED  BIOLOGY 

pods  live  in  the  sea  and  have  in  the  respiratory  chamber 
feathery  gills,  which  absorb  oxygen  from  the  water  and  give 
off  carbon  dioxide. 

Very  generally  the  hind-end  of  the  foot  of  sea-snails  bears 
a  hard  plate  which  closes  the  mouth  of  the  shell  when  the  foot 
is  retracted. 

In  most  gasteropod  shells  the  spiral  is  dextral ;  that  is, 
starting  at  the  apex  and  following  the  whorls  as  down  a  spiral 
staircase  one  would  continually  turn  to  the  right;  but 
some  snails  are  sinistral  (to  the  left).  Two  of  our  common 
pond-snails  (Planorbis  and  Physa)  have  this  peculiarity. 
The  significance  of  the  direction  of  the  spiral  is  unknown. 

The  covered  shells  of  the  garden-slugs  are  colorless. 
Cowries  are  polished  on  the  outside  because  folds  of  the 
mantle  cover  and  secrete  pearly  substance  on  the  outside 
when  the  animal  is  expanded  ("  out  of  its  shell ").  In 
most  gasteropods  the  shell  is  covered  with  an  epidermis,  but 
this  gets  rubbed  off  as  the  shell  grows  old. 

Ridges  and  spines  on  univalve  shells  are  common.  Con- 
tinuous ridges  or  lines  extending  spirally  around  the  shell  from 
its  mouth  to  apex  are  due  to  constant  and  similar  ridges  on 
the  edge  of  the  secreting  collar.  Ridges  or  rows  of  spines 
parallel  with  the  mouth  of  the  shell  and  the  lines  of  growth 
are  secreted  periodically  (example,  harp-shell).  Sometimes 
a  wide  lip  at  mouth  of  the  shell  is  formed  only  when  the 
shell  is  full  grown,  and  such  a  shell  will  have  no  such  ridges  as 
in  a  harp-shell. 

341.  The  Highest  Mollusks.  —  The  squids,  octopus 
("  devil-fish  "),  and  nautilus  represent  the  highest  forms  of 
mollusks.  All  of  them  are  marine. 

The  common  squid  (Fig.  146)  has  a  cylindrical  body, 
tapering  to  a  point  at  the  hind  end.  The  movable  head 
has  a  group  of  ten  arms  surrounding  the  mouth,  and  the 
arms  have  peculiar  suckers  adapted  to  holding  prey.  A 
large  eye  is  on  either  side  of  the  head. 


THE  SHELL- ANIMALS 


413 


At  the  end  of  the  body  is  a  pair  of  broad  fins  used  for 
swimming  slowly  forward.  On  the  lower  side  of  the  neck  is 
a  siphon  or  funnel  from  which  water  may  be  ejected  with 
such  great  force  as  to  drive  the  animal  rapidly  backward. 
At  the  same  time  it  may  eject  an  inky  fluid,  so  as  to  cloud 
the  water.  From  a  similar  ink  of  a  cuttle- 
fish India  ink  or  sepia  was  originally  made. 

The  cavity  out  of  which  the  head  pro- 
trudesjs  the  mantle-cavity,  and  the  outer 
surface  or  "  skin  "  of  the  body  is  the  mantle. 
It  contains  pigment  spots  of  various  colors, 
which  are  very  changeable  during  life. 

Externally  there  is  no  evidence  of  a  shell, 
but  beneath  the  "  skin  "  on  the  upper  side 
is  a  quill-like  structure  (the  pen)  four  to  six 
inches  long.  The  same  structure  in  cuttle- 
fishes is  the  cuttle-bone  sold  for  use  of  caged 
birds.  Comparative  studies  have  shown 
that  the  pen  and  cuttle-bone  represent 
shells  such  as  that  of  the  chambered 
nautilus. 

Giant  squids  with  arms  forty  feet  long 
and  with  eyes  a  foot  in  diameter  have  been 
captured. 

The  octopus  or  devil-fish  has  a  structure  similar  to  a  squid ; 
but  its  body  is  shorter,  there  are  eight  arms  and  there  are  no 
fins.  Some  species  are  of  enormous  size,  but  not  large  enough 
to  attack  a  ship,  as  described  in  old  books  of  fiction.  Speci- 
mens weighing  two  hundred  pounds  and  with  arms  five  feet 
long  have  been  seen. 

The  chambered  or  pearly  nautilus,  made  famous  in  the 
poem  by  Oliver  Wendell  Holmes,  is  similar  to  an  octopus  or 
squid,  but  develops'  a  many-chambered  shell.  It  lives  in 
the  outer,  largest  chamber,  and  from  time  to  time  secretes 
partitions  across  the  shell  so  as  to  leave  behind  an  out- 


FIG.  146.  Male 
squid,  a,  arms — 
two  long  ones  in 
male ;  e,  eye ;  i, 
opening  to  siphon ; 
/,  fin.  (After 
Verrill.) 


414  APPLIED  BIOLOGY 

grown  chamber.  It  lives  in  the  Indian  and  Pacific  oceans, 
usually  in  deep  water.  Some  specimens  have  been  found 
floating  on  the  surface,  and  so  Holmes's  reference  to  sailing 
may  not  be  entirely  poetic  license. 

The  paper  nautilus  (Argonaut)  is  a  relative  of  the  octopus. 
Only  the  female  bears  the  delicate,  single-chambered  shell, 
perfect  specimens  of  which  are  much  prized  by  collectors. 
According  to  an  old  fiction,  the  argonaut  did  not  secrete 
the  shell  but  stole  it  from  some  other  mollusk  in  order  to 
use  it  as  a  boat  in  which  to  sail,  using  its  two  expanded  arms 
as  sails.  Now  it  is  known  that  the  two  arms  help  secrete 
the  shell  and  hold  it  over  the  animal.  Very  little  is  known 
about  the  habits  of  this  remarkable  animal.  (Look  up 
the  classical  story  of  the  argonauts.) 

All  these  highest  mollusks  are  exceedingly  voracious  and 
destroy  large  numbers  of  fishes,  other  mollusks,  and  crus- 
taceans. The  squids  can  swim  fast  enough  to  pursue  schools 
of  fishes.  A  species  of  European  octopus  frequently  enters 
the  traps  set  for  lobsters  and  crabs,  and  the  fishermen  find 
only  mangled  remains  of  the  crustaceans. 

The  squids,  octopus,  cuttle-fish,  and  nautilus  represent  a 
class  of  mollusks  known  as  cephalopods  (Cephalopoda), 
meaning  head-foot,  referring  to  the  fact  that  in  the  embryo 
the  head  develops  from  tissue  which  forms  the  foot  in  other 
classes  of  mollusks. 

342.  Economic  Relations  of  Mollusks.  —  The  value  of 
the  oyster  industry  is  enormous  in  America  and  in  Europe. 
Studies  by  zoologists  connected  with  government  laboratories 
have  vastly  improved  methods  of  propagating,  and  have  made 
it  possible  to  raise  oysters  where  they  do  not  naturally 
grow.  Oyster  beds  are  now  regularly  leased  by  states  to 
oyster-men,  and  oysters  are  artificially  "  planted."  In  many 
places  it  is  necessary  to  rake  the  sea-bottom  with  steam 
dredges  annually  in  order  to  bring  the  oysters  to  the  surface 
and  free  them  from  the  destructive  starfishes.  The  egg- 


THE  SHELL-ANIMALS  415 

laying  months  are  May  to  August,  and  the  popular  saying 
that  oysters  are  not  edible  except  in  the  months  with  the 
letter  "  r  "  in  their  names,  i.e.,  September  to  April  inclusive, 
is  connected  with  the  fact  that  in  the  months  without  "  r" 
the  animals  are  likely  to  be  filled  with  eggs.  The  oysters  are 
the  most  valuable  mollusks,  and  the  business  of  raising  oysters 
is  worth  millions  of  dollars  a  year.  From  Chesapeake  Bay 
alone  more  than  twenty-five  million  bushels  of  oysters  are 
marketed  annually. 

The  marine  clams  and  scallops  are  of  great  importance, 
and  attempts  are  now  being  made  to  cultivate  them.  Unless 
this  becomes  commercially  successful,  the  natural  supply 
will  soon  be  exhausted.  Such  investigations  are  under  the 
auspices  of  the  United  States  Bureau  of  Fisheries  and  of 
certain  state  experiment  stations;  and  these  scientific 
institutions  deserve  the  necessary  financial  support. 

Certain  large  land-snails  have  long  been  esteemed  as 
delicacies.  There  were  snail-gardens  in  Roman  times,  but 
now  the  snails  are  widespread  pests  in  vineyards  and  gardens 
of  Europe. 

Preparations  from  snails  were  once  used  for  coughs,  con- 
sumption, malaria,  asthma,  dropsy,  and  almost  all  other 
diseases.  In  some  rural  regions  of  England  people  still 
believe  that  snails  are  of  medicinal  value. 

Numerous  marine  snails  (gasteropods)  are  used  as  food  in 
various  parts  of  the  world.  Squids  and  cuttle-fishes  are 
eaten  by  poor  people  in  some  countries. 

Sepia  or  Indian  ink  has  been  mentioned  as  obtained  from 
cuttle-fishes.  The  famous  Tyrian  purple  once  used  for 
coloring  royal  robes  came  from  another  mollusk. 

Shells  of  mollusks  are  of  great  ornamental  value.  They 
have  long  been  sought  by  conchologists  (collectors  of  shells), 
and  more  than  $100  has  often  been  paid  for  a  single  rare 
specimen. 

Natives  of  the  South  Sea  islands  use  shells  for  a  great 


416  APPLIED  BIOLOGY 

variety  of  purposes,  ornamental  and  useful.  In  some  bar- 
baric tribes  of  Central  Africa  shells  still  pass  as  money,  and 
the  "  money  cowry  "  is  the  standard  currency.  The  North 
American  natives  ("  Indians  ")  once  cut  cylindrical  beads 
from  the  purple  spots  on  shells  of  the  hard-shelled  clam 
(Venus),  and  this  was  the  "  wampum  "  mentioned  in  early 
colonial  history  of  the  United  States. 

Civilized  men  make  extensive  use  of  shells.  Some  are 
used  for  inlaying  and  for  other  ornamental  work.  Vast 
quantities  are  used  for  pearl  buttons.  Pearls  of  great  value 
are  each  year  collected  from  clams  of  various  species.  In 
North  America  many  pearls  are  found  in  the  rivers  of  the 
Mississippi  system  and  in  Lower  California. 

Reference :  Roger's  "  The  Shell  Book." 

Important  Classes  of  Mollusca 

Amphineura  —  chitons,  eight-valved  shell. 
Gasteropoda  —  snails,  univalved  shell. 
Lamellibranchiata  —  clams,  bivalved  shell. 
Cephalopoda  —  squids,  nautilus. 


CHAPTER  XVI 
THE   VERTEBRATES 

343.  The  Backboned  Animals.  —  Animals  which  possess 
a  dorsal  vertebral  column,  which  is  commonly  called  "  back- 
bone," are  members  of  the  great  group  Vertebrata.  They  are 
popularly  called  vertebrates  or  "  backboned  "  animals.  It 
should  be  noted  that  the  vertebral  column  in  some  of  the 
lower  fishes  is  cartilage,  not  bone;  and  hence  the  popular 
term  "  backbone "  for  vertebral  column  is  not  strictly 
accurate. 

The  most  convenient  classification  of  vertebrates  is  into 
five  classes :  Pisces  (fishes) ;  Amphibia  (frogs  and  sala- 
manders) ;  Reptilia  (reptiles) ;  Aves  (birds) ;  and  Mammalia 
(mammals). 

It  has  been'discovered  within  the  past  half  century  that  some 
existing  animals  lower  than  the  fishes  are  certainly  related 
to  the  vertebrates,  even  though  they  do  not  have  a  back- 
bone. However,  these  low  forms  do  have  a  stiffening  rod 
in  place  of  the  backbone,  and  it  is  similar  to  a  rod  which 
is  present  in  the  embryos  of  all  backboned  animals  as  the 
axis  around  which  the  backbone  develops.  This  rod  is 
called  the  chorda  dorsalis  (meaning  dorsal  cord)  or  notochord, 
and  all  animals  which  possess  it  in  any  stage  of  their  existence 
are  called  chordates.  The  recent  zoological  books  recognize 
a  Division  Chordata,  including  (1)  the  very  simple  forms  with 
the  chorda,  but  having  no  vertebral  column  and  skull,  and 
(2)  the  vertebrates.  In  this  course  we  cannot  study  in 
detail  any  of  the  simple  chordates ;  but  museum  specimens 
of  ascidians  (sea-squirts),  Amphioxus  (lancelet),  and  the 
2s  417 


418  APPLIED  BIOLOGY 

worm-like  Balanoglossus,  should  be  viewed  for  the  sake  of 
general  acquaintance.  Especially  note  the  general  appear- 
ance of  Amphioxus,  for  its  stucture  is  in  many  points  so 
near  the  vertebrates  that  some  authors  so  classify  it.  Noth- 
ing about  an  adult  ascidian  suggests  resemblance  to  verte- 
brates, but  its  larval  stage  is  a  tadpole-like  animal  with 
some  vertebrate  characteristics.  It  degenerates  when  meta- 
morphosing into  the  adult  stage.  All  these  simple  forms 


FIG.  147.  Outline  of  a  vertebrate  (rabbit) ,  showing  position  of  the  skeleton. 
Note  that  the  backbone,  which  incloses  the  spinal  cord,  is  in  the  dorsal 
part  of  the  body.  (From  Parker  and  HaswelL) 

will  be  interesting  to  the  student  who  elects  advanced  courses 
of  zoology  in  colleges,  or  who  reads  the  larger  textbooks. 

344.  General  Structure  of  Vertebrates.  —  The  frog,  al- 
ready studied,  is  a  good  example  of  the  general  plan  of  body 
in  vertebrate  animals.  The  important  points  of  structure  of 
vertebrates  are  as  follows :  (1)  dorsal  vertebral  column ; 
(2)  central  nervous  system  consisting  of  anterior  brain  in  a 
skull,  and  a  nerve-cord  (spinal-cord)  protected  by  the  ver- 
tebral column;  (3)  alimentary  canal  ventral  to  vertebral 
column,  and  heart  ventral  to  alimentary  canal;  (4)  body- 
cavity  containing  many  organs  (name  them) ;  (5)  usually 


THE  VERTEBRATES  419 

two  pairs  of  appendages  for  locomotion  (fins  in  fishes,  fore 
and  hind  legs  in  amphibians  and  many  higher  forms,  wings 
and  legs  in  birds,  arms  and  legs  in  man) ;  (6)  red  corpuscles 
in  the  blood.  Review  your  study  of  the  frog  and  compare 
with  this  list  of  important  points  of  structure. 

If  we  had  time  for  dissection  and  careful  study  of  a  fish, 
a  salamander,  a  snake,  a  turtle,  a  bird,  and  a  mammal,  we 
should  find  that  they  have  the  same  organs  and  arranged  in 
the  same  relative  positions  as  those  in  the  frog.  Such  com- 
parative study  must  be  left  for  advanced  courses  in  colleges. 

FISHES  * 

345.  External  Structure  of  a  Fish.  —  (L)   Any  available  fish  from 
the  market  should   be   examined.    Note  the  two  pairs  of  lateral 
fins ;   and  the  dorsal  and  ventral  fins  on  the  median  line.     Compare 
several  species  of  fishes,  or  pictures  of  them,  as  to  the  arrangement 
of  the  fins ;  and  especially  note  that  the  posterior  pair  of  fins  is  in 
some  fishes  near  the  anterior  pair.     Note  arrangement  of  scales. 

Examine  the  mouth  and  gills. 

Observe  (1)  the  movements  of  the  body  and  fins,  and  (2)  the 
mouth  and  gills  in  living  fishes  in  an  aquarium. 

346.  Distribution  and  Habits  of  Fishes.  —  Of  the  more 
than  13,000  species  of  fishes  now  known  to  exist,  about  one- 
fourth  are  found  in  or  near  North  America. 

A  remarkable  fact  regarding  the  distribution  of  fishes  is 
that  certain  species  live  at  various  depths  in  the  oceans  down 
to  over  2000  fathoms.  At  this  depth  there  is  complete  dark- 
ness, a  constant  low  temperature,  quiet  water,  and  the 
enormous  pressure  of  over  5000  pounds  per  square  inch  of 
skin.  To  such  conditions  the  deep-sea  fishes  are  adapted. 

*  The  singular  "fish"  is  commonly  used  for  singular  and  plural  when 
referring  to  one  or  more  specimens  of  a  single  species  of  fish,  e.g.,  we  may 
say  correctly  "a  ton  of  common  codfish"  which  might  include  hundreds  of 
individuals  of  the  common  species.  The  plural  word  "  fishes  "  refers  to  more 
than  one  species,  e.g.,  "there  are  at  least  140  species  of  codfishes  among  the 
more  than  13,000  different  species  of  fishes  now  known." 


420  APPLIED  BIOLOGY 

The  internal  pressure  of  gases  balances  the  enormous  external 
pressure;  but  if  brought  to  the  surface,  they  are  killed  by 
the  sudden  expansion  of  internal  gases. 

The  spawning  habits  of  some  fishes  are  most  remarkable. 
Some  salmon  ascend  the  Columbia  River  from  the  sea  for 
more  than  a  thousand  miles,  at  an  average  rate  of  3  to  4 
miles  a  day.  After  depositing  eggs  and  sperm-cells,  they  all 
die;  and  no  old  fish  lives  to  lead  the  young  ones  down 
stream  to  the  sea.  This  single  spawning  occurs  after  the 
salmon  are  at  least  three  years  old.  It  is  not  true,  as  some- 
times stated,  that  they  always  go  back  to  the  river  where 
they  were  hatched ;  but  they  probably  do  not  go  far  from  the 
mouth  of  the  river,  and  hence  are  likely  to  ascend  the  same 
river  when  fully  developed  and  ready  to  spawn. 

The  shad  of  our  Atlantic  coast  is  another  example  of 
a  fish  that  ascends  rivers  to  spawn;  but  this  fish  lives  to 
return  to  the  sea  after  spawning. 

The  common  river  eels  migrate  downstream  in  the  fall  to 
spawn  in  the  sea,  and  after  spawning  in  deep  water,  the  old 
eels  die.  Hence  adult  eels  never  migrate  upstream.  In 
spring,  vast  numbers  of  young  eels,  about  one  year  old, 
appear  below  dams  and  waterfalls.  A  female  32  inches  long 
may  have  more  than  ten  million  eggs.  The  life-history  of 
eels  was  a  complete  riddle  until  about  twenty  years  ago, 
when  it  was  found  that  the  eggs  are  laid  in  deep  sea-water. 

347.  Types  of  Fishes.  —  There  are  four  distinct  types  of 
fishes.  Specimens  should  be  examined,  if  possible. 

(1)  The  hag-fishes  and  lampreys  are  distinguished  from 
other  fishes  by  the  sucker-like  mouth  by  which  they  attach 
themselves  to  other  fishes.  The  hag-fishes  are  even  able  to 
bore  into  the  bodies  of  their  hosts.  The  lampreys  are  found 
in  lakes,  rivers,  and  seas  of  temperate  regions.  On  the  coast 
of  California  a  species  of  hag-fish  causes  much  damage  by 
taking  baited  hooks,  entangling  fishermen's  lines,  and  boring 
into  captured  fish.  On  the  Atlantic  coast  they  are  not  so 


THE   VERTEBRATES  421 

abundant.  Some  of  the  lampreys  attain  a  length  of  over 
three  feet.  They  are  excellent  as  food  fishes,  especially  in 
Europe. 

(2)  The  fishes  with  skeletons  of  cartilage  include  the  sharks, 
dog-fishes,  and  rays.     They  are  found  in  sea-water.     Some 
sharks  have   been   caught   at   depths   below  500  fathoms. 
Their  flesh  is  not  esteemed  as  food. 

Dog-fishes  are  shark-like.  Saw-fishes  have  shark-like 
bodies  with  a  saw-like  structure  on  the  snout.  Sword- 
fishes  are  similar,  but  with  a  sword  in  place  of  the  saw. 
Sting-rays  have  a  caudal  spine,  which  may  cause  severe 
wounds.  Torpedoes  have  electric  organs,  some  of  which 
can  disable  a  man.  Skates  are  harmless. 

Specimens  of  the  fishes  named  above  should  be  viewed  at 
some  museum ;  or,  at  least,  examine  pictures  in  encyclopedias 
and  zoological  books. 

(3)  Most  important  commercially  and  most  numerous  in 
species  and  individuals  are  the  bony  fishes.     The  sturgeons 
and  gar-pikes  are  the  lowest  examples.     They  have  large 
plate-like  scales  which  form  a  strong  armor  over  the  skin. 
The  sturgeons  are  large  fishes ;  and  their  flesh  is  valuable  as 
food,  and  their  ovaries  are  made  into  caviare.     They  live 
in  seas  and  rivers.     The  gar-pikes  of  North  American  rivers 
are  of  no  value  as  food. 

The  vast  majority  of  existing  fishes  in  seas  and  fresh  water 
are  of  the  type  represented  by  such  common  fishes  as  cod, 
perch,  minnow,  gold-fish,  whitefish,  bass,  pickerel,  salmon, 
trout,  carp,  mackerel,  and  halibut. 

The  skin  of  many  of  the  bony  fishes  is  beautifully  colored, 
and  some  can  change  their  color  quickly.  Scales  are  absent 
in  most  eels  and  cat-fishes.  Many  bony  fishes  are  curiously 
modified,  as  the  sea-horse,  flying-fishes,  sword-fishes,  sea- 
robin,  toad-fish,  blind-fishes,  etc.  For  accounts  of  these, 
see  encyclopedias,  or  Jordan  and  Evermann's  "American 
Fishes." 


422  APPLIED  BIOLOGY 

(4)  The  lung-fishes  of  South  America,  Africa,  and  Australia 
deserve  mention  because  they  have  gills  and  also  primitive 
lungs  able  to  breathe  air.  This  adapts  them  to  life  in  places 
where  rivers  are  muddy  or  dry  in  certain  seasons.  The 
American  and  African  forms  are  true  mud-fishes  and  bury  in 
mud  at  the  beginning  of  the  dry  season.  These  fishes  have 
attracted  much  attention  from  zoologists  because  they  sug- 
gest a  connection  between  fishes  and  amphibia. 

Important  Groups  of  Fishes 

1.  Cyclostomata  —  sucker-mouth  (hag-fish,  lamprey). 

2.  Elasmobranchii  —  cartilaginous    skeleton     (sharks,     rays,     and 

skates). 

3.  Teleostomi  -  bony  skeleton  {  ganoids  (sturgeon) 

[Teleosts  (cod,  perch,  etc.). 

4.  Dipnoi  —  lung-fishes. 

348.  Economic  Value  of  Fishes.  —  A  few  examples  of 
fishes  which  are  important  in  the  human  food-supply  will 
suggest  the  enormous  total  value  of  this  group  of  animals. 

Salmon  worth  more  than  $13,000,000  are  annually  caught 
on  the  Pacific  Coast  of  North  America,  nearly  one-third  of 
these  from  Puget  Sound  and  Columbia  River.  The  average 
weight  of  a  full-grown  salmon  of  the  Columbia  River  species 
is  over  twenty  pounds,  and  individuals  have  weighed  100 
pounds. 

Herrings  are  probably  the  most  valuable  food  fishes  in 
the  world.  Huxley  estimated  that  three  billion,  each  aver- 
aging half  a  pound  in  weight,  are  caught  in  the  North  Sea 
and  Atlantic  annually ;  and  this  is  now  too  low  an  estimate. 
They  swim  in  enormous  groups  or  "  shoals  "  which  some- 
times extend  over  half  a  dozen  square  miles.  "  Sardines  " 
from  Maine  are  simply  small  herrings,  but  the  true  European 
sardines  belong  to  another  species. 

The  codfish  is  one  of  the  most  important  North  American 
fishes.  About  7000  men  are  engaged  in  the  fishery,  and  the 


THE   VERTEBRATES  423 

annual  catch  is  in  some  years  near  100,000,000  pounds  and 
worth  to  the  fishermen  about  $2,000,000.  The  cods  live  in 
deep  water  (20  to  100  fathoms),  and  are  captured  only  with 
baited  hooks  and  lines.  A  cod  over  six  feet  long  and  weigh- 
ing over  200  pounds  was  once  taken;  but  from  12  to  40 
pounds  are  the  sizes  usually  caught.  They  spawn  near  the 
shores  of  New  England  between  December  and  April.  The 
United  States  Fish  Commission  hatcheries  liberate  more 
than  75,000,000  young  fry  annually.  It  is  easy  to  collect  the 
cod  eggs  for  hatching,  for  in  a  20-pound  female,  the  ovaries 
(popularly  called  "  roes  ")  contain  more  than  2,500,000  eggs, 
which  are  so  small  that  a  quart  bottle  will  hold  about  335,000 
eggs.  Think  of  how  abundant  codfishes  would  be  if  all  the 
eggs  of  a  thousand  females  were  to  hatch  and  grow  to 
maturity,  and  one-half  of  these  were  to  be  equally  prolific 
females.  However,  since  cods  do  not  appear  to  be  either 
increasing  or  decreasing  rapidly,  we  are  justified  in  concluding 
that,  on  the  average,  two  eggs  from  each  female  produce 
mature  individuals  (the  two  sexes  about  equal  in  number). 
The  others  are  destroyed  by  enemies  or  die  from  diseases. 
This  is  a  good  illustration  of  the  intensity  of  the  struggle  for 
existence,  which,  to  a  great  extent,  affects  all  animals  and 
plants  (§  499). 

Next  to  the  Columbia  salmon  and  the  cod,  the  shad  is  the 
most  important  fish  caught  in  waters  of  North  America.  It  is 
captured  in  the  spring  when  it  ascends  rivers  to  spawn.  The 
annual  catch  is  about  14,000,000  fish,  weight  50,000,000 
pounds,  and  worth  more  than  $1,600,000.  The  fact  that  the 
shad  is  taken  only  at  the  spawning  season  would  long  ago 
have  made  the  fisheries  unprofitable  if  the  United  States 
Fish  Commission  had  not  engaged  in  artificial  propagation. 
More  than  200,000,000  young  shad  are  annually  "  planted  " 
in  the  rivers  of  the  Atlantic  Coast.  They  have  been  intro- 
duced since  1871  on  the  Pacific  Coast,  where  they  do  not 
naturally  occur;  and  have  now  become  abundant  in  the 


424  APPLIED  BIOLOGY 

markets  of  the  west-coast  cities.  No  better  proof  could  be 
desired  as  to  the  value  of  the  work  of  the  government  in 
artificial  propagation  of  fishes.  And  this  is  only  one  of  many 
fishes  which  has  been  widely  distributed  and  made  more 
abundant  as  the  result  of  science  applied  by  the  experts  on 
fish  culture,  employed  as  agents  of  the  national  and  of 
certain  state  governments. 

The  three  most  valuable  North  American  fishes  have  been 
selected  to  illustrate  this  discussion  of  economic  value ;  but 
there  are  many  others  worth  tens  of  thousands  of  dollars 
annually.  The  fact  is  that  we  have  scarcely  begun  to  learn 
the  real  value  of  fishes  as  a  source  of  meat  food.  There  are 
hundreds  of  rivers,  lakes,  and  ponds  which  might  be  stocked 
with  fish  of  selected  species  and  made  to  produce  an  abun- 
dance of  good  food,  while,  at  the  same  time,  destroying 
numerous  larvae  of  the  dreaded  mosquitoes.  It  is  certain 
that  the  successful  methods  of  artificial  hatching  and  distri- 
bution which  have  been  discovered  by  experts  in  the  govern- 
ment service  will  ultimately  make  many  useless  bodies  of 
water  profitable  to  their  owners. 

The  student  who  is  interested  in  fishes  should  refer  to 
Jordan  and  Evermann's  "  American  Food  and  Game  Fishes." 

AMPHIBIANS 

349.  The  class  Amphibia  has  been  mentioned  as  contain- 
ing the  frogs  and  toads.  It  also  includes  the  tailed  forms 
which  are  popularly  called  newts,  salamanders,  mud-puppies 
(Necturus),  water-dogs,  and  mud-eels.  Some  of  these  are 
often  mistaken  for  lizards ;  but  lizards  (e.g.,  chameleons)  are 
reptiles  with  scaly  skin,  while  the  common  amphibians  have 
smooth  skin  like  that  of  the  frog.  Some  of  the  tailed  am- 
phibians have  gills  in  the  adult  state,  and  live  in  water. 
Those  without  gills  respire  by  means  of  the  skin  when  in 
water,  and  by  lungs  and  moist  skin  when  on  land. 


THE  VERTEBRATES  425 

The  tadpoles  of  both  the  tailed  and  tailless  forms  of  Am- 
phibia are  similar ;  but,  as  already  described  in  §  61,  the  tails 
of  frogs  and  toads  are  absorbed  while  legs  are  developing  and 
the  tadpoles  are  metamorphosing  into  the  adult  state.  The 
loss  of  the  tail,  and  the  fuller  development  of  lung-breathing 
adapts  frogs  and  toads  to  living  on  land.  Toads  are  still 
better  adapted  by  their  hard  and  dry  skin,  which  enables 
them  to  live  in  places  so  dry  that  frogs  would  perish  because 
their  skin  must  be  kept  moist. 

In  geology  the  Amphibians  are  noteworthy  because 
certain  salamander-like  forms  were  the  first  five-toed  verte- 
brates. The  remains  of  these  ancient  amphibians  have  been 
found  in  the  earliest  coal  beds  (Carboniferous  Age).  The 
present-day  types  of  amphibians  have  not  been  found  until 
late  in  geological  history  (Eocene  times). 

The  name  Amphibia  means  "both  kinds  of  life,"  referring 
to  the  fact  that  these  animals  are  both  aquatic  and  terrestrial. 

350.  Myths   Concerning  Amphibians.  —  The  skin  secre- 
tion in  some  toads  is  disagreeable  or  even  poisonous  to  their 
enemies;    but  produces  no  serious  effect  on  human  skin. 
Warts  on  small  boys'  hands  are  not  caused  by  handling 
toads.     The  truth  is  that  warts  may  be  caused  by  slight 
scratches  which  allow  dirt  to  get  into  the  skin.     Moreover, 
killing  a  toad  will  not  "  cause  your  father's  cow  to  give 
bloody  milk,"  for  a  moment's  serious  thought  shows  us  the 
absurdity  of  the  folklore  that  there  is  such  a  relation  between 
cows  and  toads.     Both  the  wart  and  the  milk  legends  are 
absolutely  unscientific.     There  are  numerous  other  absurd 
beliefs  concerning  common  animals,  and  one  who  has  read  a 
modern   textbook  on  animals  will  always  demand  proof  or 
the  authority  of  some  modern  scientific  book  before  accept- 
ing them. 

351.  Economic  Relations  of  Amphibia.  —  It  is  well  known 
that  frogs'  legs  are  eaten;    and  it  has  proved  profitable  to 
catch  frogs  for  the  market.     In  fact,  the  annual  hunting  of 


426  APPLIED  BIOLOGY 

nearly  $50,000  worth  of  wild  frogs  will  soon  exterminate  them 
unless  "  frog-farms  "  are  more  extensively  developed  in  the 
near  future.  But  the  chief  value  of  our  common  amphibians 
is  in  that  they  destroy  thousands  of  insects.  The  common 
toad  is  a  valuable  inhabitant  of  gardens.  See  the  bulletin 
on  the  American  toad  which  is  published  (free)  by  the 
United  States  Department  of  Agriculture,  and  also  see  a 
chapter  in  Hodge's  "  Nature  Study  and  Life."  We  ought  to 
have  laws  protecting  toads  as  well  as  birds,  but  until  we  get 
such  laws  we  must  depend  upon  the  good  sense  and  fair  play 
of  intelligent  people  who  are  informed  concerning  the  value 
of  toads.  Frogs  are  likewise  useful  as  destroyers  of  insects, 
but  have  not  attracted  so  much  attention  as  the  toad,  which 
is  able  to  go  far  from  bodies  of  water.  They  could  be  kept 
in  gardens  without  small  ponds. 

The  species  of  tailed  amphibia  which  spend  much  time  on 
land  are  probably  useful  destroyers  of  insects. 

Important  Orders  of  Amphibia 

Urodela  —  with  tail,  usually  two  pairs  of  limbs,  and  may  have  gills 

in  adult.     Examples  :   newt,  salamander. 
Anura,  or  Batrachia  —  no  tail  and  no  gills  in  adults.     Two  pairs  of 

limbs.     Frogs  and  toads. 
For  interesting  reading  and  reference  :  Diekerson's  "  Frog  Book." 

REPTILES 

352.  The  class  Reptilia  contains  four  types  represented 
by  the  existing  lizards,  snakes,  turtles,  and  crocodiles,  of 
which  there  are  about  3500  living  species.  All  these  types 
of  reptiles  have  scales  on  the  skin.  In  turtles  the  scales  are 
large  plates  (e.g.,  the  valuable  tortoise  shells),  which  form 
the  outer  surface  of  a  box-like  shell.  Typically  the  reptiles 
have  four  legs  with  five  toes  each.  The  legs  have  degenerated 
in  snakes,  but  certain  pythons  and  boas  possess  rudiments  of 
the  hind  pair  of  legs.  Colors  are  well  developed  in  the  skin 


THE  VERTEBRATES  427 

of  many  reptiles,  and  some  can  quickly  change  color  (e.g., 
chameleons). 

Most  reptiles  are  tenacious  of  life  and  many  can  exist  for 
a  long  time  without  food  and  with  limited  breathing.  A 
winter  sleep  in  cold  climates,  and  a  summer,  or  dry-season 
sleep  in  hot  climates,  is  their  rule  of  life. 

Fossil  reptiles  have  attracted  much  attention  because  of 
their  gigantic  size,  some  of  them  being  over  100  feet  long. 
Dinosaurs  (bird-like  reptiles),  Pterosaurs  (flying  reptiles), 
Ichthyosaurs  (fish-like  reptiles),  and  Plesiosaurs  (lizard-like), 
are  some  of  the  fossil  reptiles  commonly  seen  in  great  mu- 
seums. So  abundant  were  these  and  other  reptiles  that  one 
stage  of  geological  history  has  been  called  the  Age  of  Reptiles. 
In  North  America  the  best  specimens  of  fossil  reptiles  are 
found  in  Wyoming  and  adjoining  states. 

353.  Economic  Relations  of  Reptiles.  —  Most  lizards  are 
harmless,  and  may  be  useful  as  destroyers  of  insects.  Igua- 
nas and  other  large  species  are  hunted  for  their  flesh. 
The  Gila  monster  of  Arizona  may  sometimes  inflict  a  poison- 
ous bite.  Brilliantly  colored  lizards  are  often  kept  as  pets ; 
they  should  be  fed  insects,  and  not  starved  on  sugar  and 
water.  (Why?) 

Many  turtles  are  valuable  as  human  food.  Terrapin 
turtles  are  now  so  very  high  in  price  that  "  terrapin-farms  " 
are  profitable. 

Snakes  feed  exclusively  on  living  animals,  and  hence  may 
be  more  or  less  harmful  from  our  human  viewpoint,  espe- 
cially those  which  destroy  insectivorous  birds  and  frogs. 
Poisonous  snakes  belong  to  many  different  families.  The 
American  moccasins,  rattlesnakes,  and  copperheads,  and 
the  Old  World  cobra,  adders,  and  vipers  are  the  most  poison- 
ous reptiles.  Official  figures  show  that  in  India  alone  more 
than  20,000  people  die  annually  from  snake  bites,  but  there 
are  few  fatal  cases  in  the  United  States.  Pythons,  boas, 
and  American  blacksnakes  are  examples  of  snakes  which 


428  APPLIED  BIOLOGY 

kill  animals  by  constricting.  Most  of  the  snakes  of  the 
United  States  are  not  poisonous.  Snake  poisons  are  secreted 
by  glands  in  the  upper  lips,  and  the  "  poison  fangs  "  are  upper 
teeth  with  grooves  or  tubes  for  conveying  poison  beneath 
the  skin  of  the  victims. 

The  remedies  for  snake  bites  are  briefly  as  follows :  — 
(1)  Place  tight  ligature  on  arm  or  leg  to  prevent  circula- 
tion of  blood;  (2)  enlarge  by  cutting  the  punctures  made 
by  fangs  in  order  to  drain  away  as  much  poisoned  blood  as 
possible;  (3)  wash  out  cuts  with  wine-colored  solution  of 
potassium  permanganate;  (4)  take  very  small  doses  of 
alcoholic  stimulants,  enough  to  cause  increased  pulse-beats ; 
larger  doses  are  dangerous ;  (5)  consult  a  good  surgeon,  be- 
cause blood  poisoning  may  result  from  the  wound,  and  also 
it  may  be  necessary  to  have  stimulants  and  anti-venomous 
serum  injected  hypodermically ;  (6)  keep  wound  covered 
with  a  cloth  or  cotton  wet  with  some  antiseptic  solution. 

Alligators  and  crocodiles  are  sometimes  dangerous  to  man. 
Their  tough  skin  is  used  for  leather.  In  their  embryonic 
development,  all  crocodiles,  alligators,  turtles,  most  snakes 
and  lizards  are  oviparous.  Some  snakes  and  lizards  retain 
the  eggs  in  the  oviducts  until  development  is  completed, 
that  is,  they  are  viviparous. 

Important  Groups  of  Living  Reptiles 

Lacertilia  —  lizards  (iguana,  Gila  monster,  horned  toad,  chameleons, 
etc.). 

Ophidia  —  snakes  (vipers,  python,  boa,  water-snakes,  rattle-snakes, 
etc.). 

Chelonia  —  turtles  (tortoises,  terrapins,  soft-shelled  turtles,  box- 
turtles,  etc.). 

Crocodilia  —  crocodiles,  alligators. 

Reading  or  reference  for  students:  Dithmar's  "The 
Reptile  Book." 


THE   VERTEBRATES  429 

BIRDS 

354.  Adaptations  of  Birds.  —  No  group  of  animals  is  so 
easily  defined  as  that  of  the  birds,  for  even  a  small  child 
knows  that  an  animal  with  feathers  is  a  bird. 

Their  most  important  adaptations  are  those  connected 
with  fitting  the  wings,  legs,  and  bills  for  locomotion  and  for 
obtaining  food.  The  whole  structure  of  birds'  bodies  is 
arranged  in  adaptation  to  flying.  The  general  outline  of 
the  body,  the  peculiar  structure  of  wings,  the  great  develop- 
ment of  internal  air-sacs  connected  with  the  respiratory 
organs  —  all  are  specialized  with  reference  to  aerial  loco- 
motion. 

However,  adaptation  of  anterior  limbs  for  flying  is  not 
limited  among  vertebrates  to  birds,  for  there  were  ancient 
flying  reptiles  (Pterodactyls),  and  bats  are  flying  mammals. 
The  so-called  "  flying  fishes  "  do  not  really  fly,  but  simply 
use  their  large  fins  for  gliding  through  the  air  for  relatively 
short  distances  when  they  leap  from  the  water. 

Bats  and  birds,  then,  are  the  only  living  vertebrates 
able  to  fly,  and  the  great  differences  in  their  structures 
make  it  certain  that  they  have  developed  independently. 
Only  the  birds  among  vertebrates  have  proved  perfectly 
adapted  to  life  in  the  air.  That  they  are  well  fitted '  is 
shown  by  their  success  in  developing  more  species  than  any 
other  group  of  vertebrates,  and  also  countless  numbers  of 
individuals. 

Biologists  who  have  studied  the  flight  of  birds  cannot  help 
marveling  at  their  locomotor  mechanism.  It  is  simply 
astounding  that  an  animal  as  large  as  a  homing  pigeon  can 
fly  faster  than  a  limited  express  train,  and  average  such  speed 
from  daylight  till  dark.  The  recent  attempts  at  perfecting 
flying  machines  have  made  us  wonder  more  than  ever  at  the 
flying  power  of  the  birds. 

It  is  interesting  to  note  that  the  largest  and  heaviest  birds 


430  APPLIED   BIOLOGY 

have  lost  the  power  of  flight  (ostrich  group),  and  it  has  almost 
disappeared  in  many  birds  (ducks,  geese,  turkeys,  chickens, 
swans)  which  under  domestication  have  become  much  larger 
and  heavier  than  in  the  wild  state.  The  bodies  of  eagles, 
vultures,  and  the  other  largest  flying  birds  are  not  heavy;  and 
the  appearance  of  great  size  is  chiefly  due  to  great  expanse 
of  wings.  Evidently  lightness  of  body  is  essential.  The 
air-sacs,  already  mentioned,  help  to  provide  this  in  small 
birds,  while  in  larger  birds  there  are  also  extensive  air-spaces 
in  the  bones. 

Wings  of  birds  are  specially  adapted  for  flight,  while  the 
posterior  limbs  (legs)  are  adapted  for  the  support  and  move- 
ment on  land  and  in  water.  Since  birds  are  bi-pedal  (two- 
footed),  the  legs  are  usually  attached  comparatively  far 
forward,  so  that  the  body  is  easily  balanced. 

Legs.  —  The  modifications  of  these  are  correlated  with 
the  mode  of  locomotion  and  other  uses  of  the  feet.  The 
following  important  types  may  be  noted  among  common 
birds  ;  (1)  walking  feet  (ostrich) ;  (2)  wading  feet  (herons) ; 
(3)  climbing  feet,  two  toes  forward  and  two  backward, 
(parrots) ;  (4)  birds  of  prey  (eagle) ;  (5)  for  perching 
(pigeon) ;  (6)  swimming  feet,  with  toes  partly  or  entirely 
joined  by  webs  (ducks,  geese).  Long  wading  legs  may 
have  swimming  feet,  thus  adapting  certain  birds  to  both 
wading  and  swimming.  However,  the  swimming  feet  are 
usually  on  birds  with  relatively  short  legs. 

The  beaks  of  birds  vary  extremely  in  shape,  and  are 
adapted  to  procuring  their  special  kinds  of  food.  Among  the 
most  common  forms  of  beaks,  some  are  adapted  to  hunting 
for  food  beneath  water,  as  is  the  habit  of  swimming  and 
wading  birds ;  some  are  fitted  for  eating  seeds ;  and  some 
are  for  insect  catching.  The  pelican  bill  is  adapted  both 
to  catching  fish  and  to  storing  in  the  attached  pouch.  The 
bird-of-prey  type,  for  tearing  flesh,  has  a  short,  strong,  and 
hooked  upper  bill. 


THE  VERTEBRATES  431 

355.  Feathers    of    birds    deserve    special    examination. 
They  are  closely  allied  to  scales,  and  sometimes  (e.g.,  on  wings 
of  penguins)  look  like  scales. 

(L)  Examine  a  feather.  Note  stem,  consisting  of  quill  and 
shaft.  Examine  a  stem  split  lengthwise.  The  side  branches 
of  the  shaft  are  called  barbs,  and  their  smaller  branches  are  barbules. 
Examine  with  a  hand-lens  and  note  how  the  barbs  and  barbules 
unite  to  constitute  the  vane  of  the  feather.  The  barbules  near  the 
edge  of  the  feather  on  the  side  (lower)  next  to  the  bird's  body  have 
smaller  processes  (barbicels)  with  hooks. 

The  larger  feathers  of  wings  and  tail  (contour  feathers)  have  stiff 
shaft  and  firm  vane.  Down  feathers  have  soft  shaft  and  vane  and 
no  hooks.  Hair  feathers  are  slender  shafts  with  few  or  no  barbs. 
Examine  a  bird  and  note  where  each  kind  of  feather  is  located. 
What  is  the  use  of  each  kind  ? 

A  periodical  change  (molt)  of  feathers  usually  occurs  in 
autumn,  to  replace  the  feathers  more  or  less  damaged  by 
wear.  Many  birds  also  molt  in  part  during  the  spring  when 
acquiring  the  breeding  plumage;  but  the  change  in  color 
commonly  seen  is  due  largely  to  a  change  in  the  old  feathers. 
The  new  feathers  are  formed  in  the  follicles  or  feather-tubes 
in  the  skin,  and  first  appear  as  pointed  rods  ("pin  feathers"), 
which  are  really  the  stems  inclosing  the  forming  vanes. 
Examine  such  young  feathers,  either  fresh  or  preserved  in 
alcohol. 

Color  of  birds  is  usually  in  the  feathers,  but  sometimes  in 
comb  and  wattles  of  head  and  neck.  Feather  colors  are 
usually  due  to  pigments  (blacks,  browns,  reds,  yellows, 
rarely  greens) ;  but  metallic  luster  or  iridescent  colors 
are  produced  by  the  feather  refracting  or  dispersing  light 
as  prisms  or  thin  plates  of  various  transparent  substances 
do  (e.g.,  rainbow  colors).  Blues,  violets,  and  greens  are 
commonly  due  to  a  combination  of  pigments  and  light  re- 
fraction. There  is  no  blue  pigment  in  bird  feathers. 

356.  Internal  Organs  of  Birds.  —  Time  available  for  this 
course  will  not  permit  dissection  of  a  bird,  but  students 


432  APPLIED  BIOLOGY 

who  are  interested  should  take  the  first  opportunity  to 
examine  the  chief  organs  in  a  chicken  or  other  large  bird 
which  is  being  prepared  for  cooking. 

The  alimentary  canal  is  essentially  the  same  in  all  birds. 
Existing  birds  have  no  teeth,  but  some  fossil  forms  had  them. 
The  tongue  is  sometimes  specially  adapted  for  seizing  food 
(e.g.,  insect-catchers).  The  esophagus  often  has  a  crop  for 
storage  of  food.  The  muscular  stomach  (called  gizzard)  has 
walls  whose  strength  varies  with  the  food  (strong  in  grami- 
nivorous birds).  It  contains  small  stones  which  the  bird  has 
swallowed  to  aid  in  grinding  the  food.  From  the  stomach 
there  is  a  coiled  intestine  into  which  a  liver  and  a  pancreas 
pour  their  secretions.  The  posterior  part  of  the  intestine  is 
expanded  into  a  cloaca,  into  which  the  ducts  from  kidneys 
and  from  the  reproductive  organs  open. 

The  respiratory  organs  are  very  peculiar.  The  voice- 
organ  is  not  in  the  larynx,  as  in  mammals,  but  is  lower  down 
on  the  trachea  or  "wind-pipe.  This  voice-organ  (also  called 
syrinx)  is  a  complicated  structure,  especially  in  singing  birds. 
Large  air-cavities  in  the  bird's  body,  and  even  in  the  bones 
of  some,  are  connected  with  the  bronchial  tubes  leading  from 
the  trachea.  In  fact,  when  a  bird  breathes  by  dilating  its 
thorax  and  abdomen,  a -large  part  of  the  air  inhaled  rushes 
into  air-sacs  and  very  little  distention  of  the  lungs  occurs 
as  in  mammals.  This  peculiar  mechanism  is  a  more  efficient 
respiratory  apparatus  than  lungs  of  mammals,  and  the  oxy- 
gen-supply to  the  blood  is  more  complete.  This  more  rapid 
respiration  is  necessary  because  of  the  great  activity  of  birds, 
especially  in  flying  and  in  singing. 

Another  remarkable  fact  connected  with  the  respiratory 
organs  is  that  their  great  surface  eliminates  excretory  water 
and  excess  heat.  In  mammals,  this  is  accomplished  by  the 
kidneys  and  sweat-glands ;  but  bird  kidneys  do  not  eliminate 
much  water,  and  they  have  no  sweat-glands  in  their  skin. 

As  a  result  of  the  intense  respiration  and  consequent 


THE  VERTEBRATES  433 

rapid  oxidation  of  birds  their  temperature  is  higher  than 
that  of  mammals,  some  birds  reaching  110°  F.,  which  would 
be  fatal  in  a  mammal. 

357.  Classification  of  Birds.  —  No  classification  of  birds 
yet  prepared  has  been  generally  accepted,  and  no  two  books 
agree.  The  difficulty  arises  from  the  fact  that  the  12,000 
species  of  birds  are  remarkably  similar  except  in  details  of 
structure.  It  is  not  possible  in  limited  time  to  describe 
the  groups  of  birds,  and  for  general  purposes  it  is  most  con- 
venient to  name  groups  according  to  some  of  their  well- 
known  representatives,  as  in  1  to  16  below.  For  each  of 
these  groups  there  is  a  scientific  name,  for  which  see  special 
books  on  birds. 

Groups  of  Birds 

1.  Ostrich   group  —  ostriches,  emu,  cassowaries,  kiwi   (Apteryx), 

moas.    All  wingless  or  with  greatly  reduced  wings. 

2.  Loon  group  —  loons,  divers,  grebes. 

3.  Gull  group  —  gulls,,  terns,  petrels,  albatross. 

4.  Pelican  group  —  pelicans,  cormorants. 

5.  Duck  group  —  ducks,  geese,  swans. 

6.  Heron  group  —  herons,  bitterns,  storks,  flamingos,  spoon-bill. 

7.  Rail  group  —  rails,  coots. 

8.  Snipe  group  —  snipes^  plovers,  woodcock,  "killdeer." 

9.  Pheasant  group  —  pheasants,  grouse,  quails,  chickens,  turkeys. 

10.  Pigeon  group  — -  pigeons,  doves. 

11.  Eagle  group  —  eagles,  hawks,  vultures,  falcons. 

12.  Owls. 

13.  Cuckoo  group  —  cuckoos,  kingfishers. 

14.  Woodpecker  group  —  woodpeckers,  flickers. 

15.  Humming  birds. 

16.  Perching    or   song-birds,    6000   species  —  sparrows,   warblers, 

crows,  jays,  fly-catchers,  finches,  robins,  thrushes,  bluebirds, 
etc.     Includes  all  the  most  interesting  song-birds. 

A  popular  classification  arranges  birds  according  to  their 
habits  as  wading  birds,  birds  of  prey,  swimming  birds,  climb- 
ing birds,  and  perching  birds.  This  is  very  convenient  for 
most  people,  for  they  are  chiefly  interested  in  how  birds  live 

2F 


434  APPLIED  BIOLOGY 

and  affect  human  interests.  It  is  important  to  note  that 
such  a  grouping  of  birds  with  similar  habits  does^not  always 
correspond  to  a  scientific  classification  based  on  similarity 
of  both  external  and  internal  structure.  For  example,  there 
are  swimming  birds  in  groups  2,  3,  4,  5  named  above;  but 
loons,  gulls,  pelicans,  and  ducks  show  little  structural  evi- 
dence of  relationship  aside  from  their  swimming  adaptations, 
and  hence,  in  a  scientific  classification,  must  be  placed  in 
separate  groups.  However,  those  who  do  not  specialize  in 
bird  study  cannot  do  better  than  remember  the  most  familiar 
birds  either  according  to  their  habits,  or  as  associated  with 
their  relatives  mentioned  in  groups  1  to  16  above. 

358.  Instincts  of  birds  are  highly  developed.  They  have 
sharp  eyes  and  a  good  memory.  Parrots  and  ravens  show 
an  extraordinary  power  of  imitation  and  ability  to  take  some 
instruction. 

The  most  highly  developed  instincts  of  birds  are  connected 
with  migration  and  reproduction.  It  is  well  known  that 
most  North  American  birds  migrate  northward  in  spring, 
and  after  the  breeding  return  southward  in  autumn.  What 
causes  them  to  migrate,  and  especially  what  guides  them  on 
the  journey,  has  long  puzzled  naturalists.  A  vast  amount  of 
information  concerning  the  times  and  paths  of  migration  of 
many  species  in  the  Northern  Hemisphere  has  been  recorded 
in  the  special  books  of  ornithology. 

The  distance  covered  in  migrations  varies  with  species. 
Some  move  from  the  arctic  to  temperate  regions  in 
autumn,  and  return  in  spring  (e.g.,  certain  "  snow-birds  ") ; 
others  migrate  between  tropical  and  temperate  regions; 
and  still  others  go  from  tropical  to  arctic  regions.  Some 
species  migrate  »at  night,  others  by  day.  Some  migrate  at 
great  heights.  In  some  remarkable  cases,  the  young  birds 
go  southward  before  their  parents ;  and  in  other  cases,  the 
parents  go  first.  In  these  cases,  we  cannot  understand  how 
the  young  are  guided,  unless  by  a  few  old  birds  which  have 


THE   VERTEBRATES  435 

in  the  previous  year  had  experience  in  migration.  Probably 
in  most  cases  the  young  birds  hatched  in  northern  regions 
learn  the  way  southward  by  accompanying  older  birds. 

Migration  southward  is  believed  to  be  stimulated  by  ab- 
sence of  food-supply  in  winter ;  and  a  desire  to  re-occupy  their 
old  haunts  and  breeding  places  leads  to  return  in  spring. 

Some  birds  may  be  seen  at  given  localities  in  any  month 
of  the  year,  and  such  species  are  said  to  be  resident.  Many 
birds  which  are  resident  as  species  migrate  as  individuals; 
and  those  seen  in  winter  have  often  come  from  the  north, 
while  the  summer  residents  of  the  same  species  have  moved 
southward. 

The  migration  is  not  always  due  north  and  south.  Coast 
lines,  mountains,  and  great  rivers  may  cause  eastward  or 
westward  deviations.  These  are  not  yet  well  understood. 

The  instincts  connected  with  nesting  habits  of  birds  have 
long  excited  the  wonder  of  ornithologists.  Many  birds  build 
nests  exquisite  in  form.  Each  species  has  its  peculiarities 
as  to  choice  of  nesting  site.  Why  quails  nest  on  the  ground, 
swifts  in  a  chimney,  swallows  beneath  the  house-eaves,  and 
orioles  in  hanging  nests  cannot  be  satisfactorily  explained; 
and  we  must  be  content  with  saying  that  these  birds  have 
inherited  their  nest-building  instincts  from  their  ancestors. 
How  and  why  their  ancestors  learned  to  build  nests  in  certain 
ways  and  in  specially  selected  positions  is  entirely  a  mystery. 

The  instincts  connected  with  incubation  of  the  eggs  and 
caring  for  the  helpless  young  are  no  less  remarkable.  In 
some  species  the  female  broods  the  eggs  and  her  mate  brings 
food  to  her.  In  others  the  male  and  female  take  turns  in 
brooding  the  eggs.  Certain  cuckoos  and  some  other  birds 
have  the  peculiar  habit  of  avoiding  the  trouble  of  brooding 
by  placing  their  eggs  in  the  nests  of  other  birds,  which 
incubate  them.  In  many  species  both  male  and  female 
cooperate  in  the  work  of  collecting  food  for  the  young 
nestlings. 


436  APPLIED  BIOLOGY 

Chapman's  "Bird  Life"  is  an  excellent  introduction  to 
bird  study.  There  are  many  good  books  specially  adapted 
for  identification  of  birds  in  given  localities. 

MAMMALS 

359.  Characteristics.  —  Mammals    (Mammalia)    are    the 
animals  which  in  popular  language  are  termed  "quadrupeds" 
or  "  beasts."     They  are  sharply  distinguished  from  all  other 
vertebrates    by   three    characteristics :     (1)  milk-glands    or 
mammary  glands  for  supplying  food    to  the  young   (the 
name  "  mammal  "  refers  to  this  peculiarity) ;    (2)  true  hair 
composed  of  overlapping  scales  or  dry  cells ;  and  (3)  a  dia- 
phragm  dividing   the   body-cavity  into   an   anterior   (tho- 
racic) cavity  with  heart  and  lungs,  and  a  posterior  (abdomi- 
nal) cavity.     No  other  animals  have  these  characteristics, 
and  by  applying  them  we  can  quickly  decide  that  whales  are 
mammals  and  not  fishes,  and  that  bats  are  not  birds;   for 
both  whales  and  bats  have  milk-glands,  hair,  and  diaphragm. 

360.  Groups  of  Mammals.  —  It  is  easiest  to  learn  the 
chief  groups  of  mammals  by  reference  to  well-known  examples 
such  as  one  may  see  in  any  zoological  garden,  menagerie, 
or  natural-history  museum.     In  citing  examples  below,  the 
plural  (e.g.,  kangaroos)  is  used  in  most  cases  where  more  than 
one  species  is  well  known  under  the  same  popular  name. 

There  are  about  2400  species  of  living  mammals,  and  over 
3000  extinct  species. 

Orders  of  Mammals 

1.  Monotremata   (monotremes)  —  Australian  duck-bill  and  spiny 

ant-eater.  Lowest  mammals.  Oviparous.  All  higher  mam- 
mals are  viviparous. 

2.  Marsupalia  (marsupials)  —  opossums,  kangaroos,  wombats,  and 

bandicoots.  The  skin  on  ventral  side  of  abdomen  forms  a 
pouch  (marsupium)  in  which  the  very  weak  young  are  carried. 
About  180  living  species  are  known. 


THE   VERTEBRATES  437 

3.  Edentata  (edentates)  —  sloths,  armadillos,  most  ant-eaters. 

4.  Cetacea    (cetaceans)  —  whales,    porpoises,    dolphins.      All   are 

aquatic,  with  fish-like  bodies,  no  posterior  (pelvic)  limbs. 

5.  Sirenia  —  the  mantee  or  sea-cow. 

6.  Ungulata     (ungulates    or    hoofed    mammals)  —  horses,    asses, 

zebras,  tapirs,  rhinoceroses,  camels,  cattle,  sheep,  goats,  ante- 
lopes, giraffes,  deer,  pigs,  hippopotami,  elephants,  and  extinct 
mastodons.  Numerous  extinct  ungulates  are  found  as  fossils. 

7.  Carnivora     (carnivors     or     flesh-eaters)  —  cats,    hyenas,    dogs, 

wolves,  foxes,  jackals,  bears,  otters,  weasels,  seals,  walruses. 

8.  Rodentia  (rodents  or  gnawers)  —  rats  and  mice,  rabbits  and  hares, 

squirrels,  porcupines,  and  beavers.  There  are  about  1500 
species  of  rodents. 

9.  Insectivora  (insectivors)  —  moles,  shrews,  and  hedgehogs. 

10.  Cheiroptera  —  bats  and  flying-foxes.     Several  hundred  species. 

11.  Primates  —  lemurs,    marmosets,    monkeys,    baboons,   gibbons, 

orangs,  chimpanzees,  gorilla.  The  highest  family  of  this  order 
of  mammals  is  the  Hominidse,  which  includes  only  the  human 
species  (Homo  sapiens).  See  §  369. 

361.  Adaptations  of  Mammals.  —  Space  here  will  allow 
only  mention  of  some  of  the  most  remarkable  cases  of  mam- 
malian structures  which  are  specially  adapted. 

(1)  Fore  limbs  of  bats  have  been  modified  into  wings  for 
flying,  but  still  retain  the  same  number  and  arrangement 
of  bones  as  in  ordinary  five-toed  mammals. 

(2)  Posterior  limbs  of  whales  have  become  rudimentary 
in  fitting  to  aquatic  life.     The  remains  of   the   bones  are 
often  found  several  feet  beneath  the  skin.     Seals  which  live 
partly  on  land  make  little  use  of  their  small  hind  legs. 

(3)  The  feet  of  most  hoofed  mammals  show  remarkable 
adaptations.     The  typical  foot  originally  had  five  toes,  but 
some  of  the  toes  have  either  become  so  small  that  they  do 
not  touch  the  ground  or  have  disappeared  altogether.     The 
following  examples  will  illustrate  this  point.     An  elephant's 
foot  has  five  complete  toes  (or  digits)  each  with  a  hoof,  and 
all  united  by  skin.     The  wild  and  domesticated  hogs  and 
hippopotami  have  four  toes  on  each  foot,  the  first  one  (I)  on 


438  APPLIED  BIOLOGY 

the  inner  side  (thumb)  being  absent,  the  two  outer  toes  (II 
and  V)  are  short  and  do  not  touch  the  ground,  while  the 
third  and  fourth  are  the  toes  upon  which  the  animal  walks. 
In  deer  the  two  outer  toes  (II  and  V)  are  short  and  in 
some  species  absent.  In  camels  and  giraffes  there  are 
two  toes  on  a  foot,  the  third  and  fourth,  the  outer  ones 
(II  and  V)  being  absent.  In  sheep,  goats,  antelopes,  and 
cattle,  the  useful  toes  are  the  third  and  fourth,  and  the 
outer  ones  (II  and  V)  are  often  represented  by  small  hoofs. 

The  animals  mentioned  above,  except  elephant,  are  even- 
toed,  i.e.,  with  two  or  four  useful  toes  on  a  foot.  Other 
hoofed-mammals  are  odd-toed,  with  five,  three,  or  one. 

The  elephant  has  five  toes,  the  rhinoceros  three  (some  spe- 
cies with  four  on  front  feet),  the  tapir  three  on  hind  feet  and 
four  on  fore  feet,  and  the  existing  horses  have  one  toe.  The 
three  toes  on  a  tapir  or  rhinoceros  foot  correspond  to  second, 
third,  and  fourth  of  an  elephant.  The  fourth  toe  on  a  tapir's 
front  foot  is  the  fifth.  The  one  toe  or  hoof  of  the  existing 
horses  is  the  third,  that  is,  two  toes  on  either  side  have  dis- 
appeared from  the  typical  five-toed  foot. 

Along  with  the  reduction  of  toes  from  five  to  three,  two, 
or  even  one,  there  have  been  changes  in  the  bones  which 
connect  toes  to  the  legs. 

Reduction  in  the  number  of  toes  is  well  shown  by  the 
geological  history  of  the  horse,  as  shown  by  fossils  now  ex- 
hibited in  the  great  museums.  Figure  148  shows  a  series  of 
such  fossil  feet  from  a  four-toed  horse  of  Eocene  times  down 
to  the  present  one-toed  type.  A  five-toed  horse-like  animal 
has  not  yet  been  found.  It  should  be  noted  that,  as  the 
middle  or  third  toes  became  the  useful  ones,  the  reduced  toes 
were  left  as  small  bones  at  the  sides.  In  existing  horses 
there  are  two  "  splint  bones  "  which  represent  the  degen- 
erated second  and  fourth  toes  of  the  three-toed  horses. 

These  modifications  of  the  feet  of  hoofed  animals  appear  to 
be  adaptations  to  different  habits  of  life.  The  horse  is 


THE  VERTEBRATES 


439 


admirably  fitted  to  swift  running  on  a  hard  plain,  but  the 
three-toed  ancestors  would  have  been  better  adapted  to 
marshy  land.  It  is  well  known  that  our  present  horses  can- 
not travel  on  very  marshy  ground  because  the  single  hoof  is 
difficult  to  withdraw  from  mud.  This  is  why  oxen  are  used 


FIG.  148.  Fore  foot  of  ancestral  forms  of  the  horse.  Toes  are  numbered  in 
comparison  with  a  five-toed  animal  in  which  I  is  the  inner  toe  and  V 
the  outermost.  Only  III  remains  in  the  present-day  horse  (6) ;  but  there 
were  four  toes  on  the  earliest  known  ancestors.  (From  Wiedersheim.) 

in  plowing  and  otherwise  working  on  very  soft  soil,  for  their 
two-toed  feet  are  easily  withdrawn  from  deep  mud.  This 
leads  us  to  think  that  the  three-toed  horses  of  the  past  ages 
could  have  traveled  over  marshy  land  as  oxen  and  rhinoce- 
roses now  do. 

The  two-toed  animals  appear  to  have  special  advantages 
in  rough  and  mountainous  countries  as  well  as  in  low  and 
swampy  regions.  A  deer  or  an  antelope  can  run  swiftly  on  a 
hard  plain,  but  its  feet  are  also  adapted  to  other  conditions 
not  suited  to  the  horse  feet.  The  feet  of  camels  are  specially 
suited  to  the  sandy  regions  where  they  occur,  for  the  two-toed 
condition  allows  considerable  speed  even  on  yielding  sands. 

Students  who  are  specially  interested  in  study  of  adapta- 
tions of  ungulate  feet  should  examine  the  illustrations  in 
such  books  on  evolution  as  Romanes,  "Darwin  and  after 
Darwin,'7  Vol.  I,  Figs.  73-85. 

(4)  There  are  numerous  adaptations  of  feet  and  jaws  in 
carnivors.  Especially  noteworthy  are  the  feet  with  four  or 


440  APPLIED  BIOLOGY 

five  well-formed  toes  with  claws,  and  the  large  canine  teeth. 
A  rudiment  of  the  toe  which  does  not  touch  the  ground  may 
be  seen  in  dogs.  It  is  the  first,  i.e.,  towards  middle  of  the 
body.  In  seals  and  walruses  the  limbs  are  especially  adapted 
for  swimming. 

(5)  The  head  and  fore  feet  of  moles  are  well  adapted  for 
burrowing  in  soil. 

(6)  The  trunk  (proboscis)  of  elephants  is  an  extension 
of  the  nose.     The*  tusks  of  ivory  are  enormously  developed 
teeth  (incisors)  of  the  upper  jaw.     The  tusks  of  the  walrus 
are  upper  canine  teeth. 

(7)  Nearly  all  the  apes  and  monkeys  are  well  adapted  to 
arboreal  life.     This  is  obvious  when  we  watch  the  ways  in 
which  they  use  their  hands  and  feet  in  climbing. 

(8)  The  pouch  (marsupium)  of  kangaroos  and  opossums 
is  a  special  adaptation  for  protecting  the  young,  which  are 
born  in  an  exceedingly  undeveloped  condition.     Inside  the 
pouch  are  milk-glands  for  supplying  food. 

(9)  The  horns  and  antlers  of  cattle,  sheep,  goats,  and  deer 
are  adaptations  for  defense.     Antlers  of  male  deer  are  shed 
annually,  and  new  ones  grow  rapidly.     Both  male  and  female 
reindeer  have  antlers.     The  hollow  horns  of  cattle,  sheep, 
goats,  and  antelopes  are  not  shed. 

(10)  The  skin  of  rhinoceroses,  elephants,  and  hippopotami 
is  enormously  thickened,  making  them  difficult  to  kill  even 
with  powerful  guns. 

(11)  The  mass  of  long  hairs  forming  a  mane  along  the  dorsal 
surface  of  necks  of  horses  and  their  near  allies  protects  against 
bites,  for  horses  can  fight  more  viciously  with  their  jaws  than 
with  their  hoofs. 

(12)  The  teeth  of  the  rodents  are  specially  modified  for 
gnawing  wood,  nuts,  and  other  plant  materials. 

(13)  The  skin  of  many  carnivors  is  in  winter  well  protected 
by  close-set  hairs  which  form  furs.     Examples  of  great  'com- 
mercial value  are  seals,  foxes,  minks,  bear.     The  hairs  of 


THE  VERTEBRATES  441 

some  hoofed  animals  develop  as  a  woolly  coat,  as  in  sheep, 
goats,  American  bison,  and  Persian  lamb. 

(14)  Adaptations  for  special  methods  of  locomotion  are 
very  common  among  mammals.  Examples  are  :  kangaroos 
for  jumping,  sloths  for  hanging  from  under  side  of  branches 
of  trees,  horses  for  running  on  solid  ground,  seals  and  whales 
for  swimming,  bats  for  flying,  and  many  mammals  of  different 
orders  for  climbing. 

Many  other  interesting  adaptations  of  mammals  may 
be  found  described  in  books  on  natural  history. 

362.  Economic  Relations  of  Mammals.  —  No  other  class 
of  animals  approaches  that  of  the  mammals  in  economic 
importance.  The  truth  of  this  statement  will  be  obvious 
to  any  one  who  considers  the  vast  monetary  value  of  the 
common  domesticated  mammals — horses,  cattle,  sheep,  pigs, 
goats,  and  dogs.  Also,  in  some  countries,  camels,  elephants, 
llamas,  and  reindeer  are  important  domesticated  mammals. 

The  domesticated  species  are  useful  in  (1)  the  human  food- 
supply,  and  (2)  as  beasts  of  burden. 

Many  wild  mammals  are  also  useful  to  man.  The  most 
valuable  of  these  are  the  fur-bearers  (chiefly  carnivors, 
such  as  seals,  foxes,  mink,  bear).  Whales  have  long  been 
hunted  for  the  whale-bone  obtained  from  their  jaws  and  the 
sperm-oil  from  their  "  blubber."  A  large  whale  of  one  species 
may  yield  over  $10,000  worth  of  whalebone  and  three  hun- 
dred barrels  of  oil.  Elephants  and  walruses  have  been  ruth- 
lessly hunted  for  the  ivory  of  their  tusks.  Sea-cows  are 
hunted  for  their  flesh,  oil,  and  hides.  Beavers  have  been 
nearly  exterminated  because  of  their  valuable  skins. 

Modern  methods  have  made  it  possible  to  utilize  every 
particle  of  mammals  slaughtered  for  human  food.  In  ad- 
dition to  the  meat  obtained,  the  poorer  qualities  of  fat  are 
made  into  soap  ;  the  horns  into  combs ;  the  hoofs  into  glue ; 
the  best  hair  into  packing  for  many  purposes  and  the  poorer 
grade  is  used  in  plastering  walls  of  buildings ;  gelatin  is  made 


442  APPLIED  BIOLOGY 

from  tendons ;  leather  from  the  derails  of  the  skin ;  lean  scraps 
and  blood  are  dried  to  make  foods  for  poultry  and  other 
animals ;  bones  are  made  into  hundreds  of  useful  articles  and 
the  best  small  pieces  are  ground  into  bone-meal  for  feeding 
poultry;  and  any  particles  of  bone,  blood,  or  flesh,  not 
usable  in  other  ways,  is  dried  and  pulverized  to  make  com- 
mercial fertilizers  for  agricultural  use. 

The  dog  was  the  first  domesticated  animal,  and  it  is  inter- 
esting to  note  that  primitive  dogs  were  probably  chiefly  kept 
as  pets  and  companions,  just  as  many  of  our  modern  dogs 
are  to-day.  The  development  of  such  uses  as  hunting,  guard- 
ing flocks  of  sheep,  and  drawing  sledges  and  carts  seems  to 
have  come  after  the  dog's  masters  began  to  emerge  from  the 
lowest  barbarism. 

Among  very  injurious  mammals  are  numerous  species  of 
rodents  (e.g.,  rats,  mice,  gophers,  prairie-dogs,  certain 
squirrels,  rabbits  in  Australia) ;  some  carnivors  (e.g.,  the 
dangerous  cat-like  species,  the  weasel-like  forms,  the  bears 
and  wolves)  ;  and  fruit-bats. 

Some  mammals  which  are  insect-eaters  are  indirectly  bene- 
ficial. Many  bats,  ant-eaters,  and  the  moles  are  examples; 
but  owing  to  their  subterranean  habits  the  moles  do  much 
damage  to  roots  among  which  they  burrow  in  search  of  larvae 
of  insects. 

References :  Concerning  the  injurious  mammals  (rodents, 
wolves,  etc.)  there  are  many  pamphlets  issued  by  the  United 
States  Department  of  Agriculture.  Two  of  the  most  inter- 
esting books  dealing  with  domesticated  mammals  are  Shaler's 
"  Domesticated  Animals,"  and  J.  G.  Wood's  "  Dominion 
of  Man." 

LIFE-HISTORIES  OF  VERTEBRATES 

363.  Embryology  of  Lower  Vertebrates.  —  The  lesson  on 
development  of  the  frog'sxegg  (§§  56-63)  should  be  reviewed 
before  continuing  the  study  of  the  present  chapter. 


THE   VERTEBRATES 


443 


In  all  species  of  vertebrates  there  are  both  male  and  female 
individuals ;    and  new  individuals   always   originate   from 
egg-cells  fertilized  by 
sperm-cells.      Asexual 
reproduction  and  par- 
thenogenesis   are   not 
known    among    these 
higher  animals. 

The  eggs  of  most 
fishes  are  laid  in  shal- 
low water  in  quiet 
places,  and  sperm-cells 
discharged  into  the 
water  fertilize  the  eggs. 
The  enormous  loss  of 
eggs  and  young  fishes 
under  natural  condi- 
tions has  led  to  artifi- 
cial fish-culture  under 
the  control  of  various 
states  and  the  United 
States  Bureau  of  Fish- 
eries. The  methods  of 
hatching  are  a  scien- 
tific application  of  na- 
ture'sway.  Fishes  are 
caught  by  nets  at  the 
breeding  season,  and 
pressure  on  their  ven- 
tral surfaces  causes  ex- 
trusion of  eggs  and 
sperm-cells.  These  are 
mixed  in  water  and 

allowed  to  stand  in  shallow  pans  or  water-tight  boxes  until 
the  sperm-cells  have  by  swimming  reached  and  penetrated 


FIG.  149.  Stages  in  development  of  a  fish. 
bl,  germ-disc,  remainder  of  the  egg  is  yolk  ; 
emb,  developing  embryo  ;  ys,  yolk-sac  at- 
tached to  ventral  surface  after  hatching. 
(From  Parker  and  Haswell.) 


444  APPLIED  BIOLOGY 

the  egg-cells  (one  sperm  for  each  egg),  and  fertilized  them. 
Later,  the  fertilized  eggs  are  placed  in  boxes  arranged  so 
that  water  runs  over  them  while  they  develop.  After 
hatching,  the  young  fish  (called  fry)  are  kept  in  small  pools 
or  tanks,  where  they  are  easily  fed  and  protected  until  large 
enough  to  care  for  themselves  in  rivers  and  ponds  where  their 
enemies  live.  Of  course  many  are  killed  after  they  are 
turned  loose,  but  a  far  larger  number  of  eggs  may  develop 
into  adult  fishes  if  hatched  under  artificial  conditions  than  if 
allowed  to  be  laid  where  fishes  will  naturally  deposit  them. 

364.  Embryology  of  Higher  Vertebrates.  —  The  early 
stages  of  the  embryonic  development  of  reptiles,  birds,  and 
mammals  have  a  general  similarity  to  those  of  the  frog  (§  59) . 
In  all  cases,  the  fertilized  egg-cell  divides  into  numerous 
cells  which  then  form  the  body  of  the  embryo. 

It  is  a  significant  fact  that  there  is  great  similarity  in  the 
early  stages  of  all  vertebrates.  This  is  illustrated  by  Fig. 
150,  in  which  in  parallel  columns  are  early,  intermediate,  and 
late  embryos  of  a  fish,  a  salamander,  a  reptile,  a  bird,  and  a 
mammal.  In  the  early  stages  there  is  so  great  similarity  that 
only  specialists  in  zoology  could  distinguish  between  these 
embryos ;  but  as  development  proceeds  there  is  more  and 
more  differentiation,  and  the  final  stages  at  birth  or  hatching 
are  easily  identified  as  fish,  bird,  etc. 

One  of  the  most  remarkable  facts  connected  with  this 
similarity  of  embryos  of  different  classes  of  vertebrates  is  the 
presence  in  all  of  them  of  certain  structures  which  are  use- 
less in  the  higher  forms.  Most  striking  of  such  useless  struc- 
tures are  the  gill-slits.  In  fishes  these  openings  from  the 
pharynx  to  the  exterior  are  exits  for  water  which  enters  at 
the  mouth.  Between  the  slits  are  the  gills,  through  whose 
delicate  membranes  blood  circulates  and  exchanges  oxygen 
and  carbon  dioxide  with  the  water.  Thus  in  fishes  the  gill- 
slits  are  useful  as  part  of  the  respiratory  system. 

The  gill-slits  develop  in  very  young  embryos  of  fishes  and 


THE  VERTEBRATES 


445 


FIG.  150.     Similarity  of  early  stages  of  vertebrate  embryos,  and  later 
differentiation.     (From  Romanes,  after  Haeckel.) 


446  APPLIED  BIOLOGY 

remain  throughout  life,  functioning  as  described  above.  In 
all  amphibia  (frogs  and  salamanders)  gill-slits  are  found  in 
the  embryos  and  are  commonly  present  in  the  young  tadpoles ; 
but  in  all  adult  frogs  and  toads  and  in  most  salamanders  the 
gill-slits  are  closed  when  the  adult  stage  is  reached,  and  then 
they  breathe  by  lungs  and  skin.  Finally,  in  all  reptiles, 
birds,  and  mammals  gill-slits  develop  in  the  embryos,  but 
they  normally  close  before  hatching  or  birth.  They  are  never 
in  these  animals  of  any  possible  use  for  breathing,  for  in  the 
stage  in  which  the  gill-slits  occur  the  embryos  do  not  live 
in  water,  where  gills  could  serve  for  breathing. 

It  is  clear  why  gill-slits  develop  in  fishes  and  amphibia 
which  use  them  as  respiratory  organs;  but  obviously  this 
does  not  explain  their  presence  in  the  higher  vertebrates. 
Nor  has  any  other  physiological  explanation  been  found; 
and  the  gill-slits  appear  to  be  useless  structures.  Why  then 
do  they  develop  in  the  embryos  of  all  reptiles,  birds,  and 
mammals  ?  The  only  answer  which  is  satisfactory  to  modern 
zoologists  is  that  gill-slits  in  higher  vertebrates  suggest  that 
these  had  fish-like  ancestors  in  the  far-distant  ages,  and  that 
from  these  ancestors  the  gill-slits  have  been  inherited.  In 
short,  gill-slits  in  reptiles,  birds,  and  mammals  are  ancestral 
reminiscences. 

There  are  many  other  structures  in  embryos  of  higher 
forms  which  have  been  explained  only  on  the  ground  of 
inheritance. 

365.  Bird  Development.  —  All  species  of  birds  are  ovip- 
arous (external  development) ;  and  the  eggs  require  in- 
cubation. For  this  a  certain  temperature  is  essential  (about 
103  F.  for  hen's  eggs).  The  eggs  are  fertilized  soon  after  they 
leave  the  ovary  and  enter  the  oviduct,  and  cell-division  goes 
on  for  about  a  day  while  the  eggs  are  passing  through  the 
duct  to  the  exterior.  But  soon  after  an  egg  is  "  laid  "  it 
becomes  cooled  to  below  the  normal  temperature  and  devel- 
opment stops.  Within  a  variable  number  of  days,  the  de- 


THE   VEETEBEATES 


447 


velopment  may  start  again  if  the  egg  be  warmed  to  the  proper 
temperature.  In  natural  conditions  this  is  provided  for  by 
the  instinct  which  causes  female  birds  (sometimes  the  males) 
to  sit  on  or  brood  the  eggs.  The  feathers  prevent  rapid  loss 
of  the  heat  afforded  by  the  warm  ventral  surface  of  the  body 
of  a  brooding  bird.  This  brooding  instinct  usually  appears 
soon  after  a  female  bird  has  laid  the  eggs  which  in  a  given 
season  have  developed  in  the  ovary.  In  many  wild  birds 
such  a  season  of  egg-laying  comes  only  once  a  year ;  in  some 
species  two  or  three  broods  of  eggs  may  be  laid  in  a  summer ; 
and  the  well-fed  domesticated  hen  may  lay  from  100  to  more 
than  200  eggs  per  year,  if  not  allowed  to  waste  time  and 
energy  by  brooding  after  each  set  of  10  to  20  eggs,  as  they 
instinctively  do.  The  eggs  of  birds  are  large  because  they 
have  a  great  store  of  food  (yellow  "  yolk,"  and  the  "  white  " 
or  albumen)  for  nourishment  of  the  embryo  during  the  de- 
velopment. The  eggs  in  the  ovaries  of  young  birds  are  small 
spherical  cells,  but  as  they  mature  the  storage  of  food  causes 
enlargement.  For  exam- 
ple, in  an  ordinary  hen's 
egg  the  "  yolk  "  with  its 
inclosing  yolk-membrane 
is  about  one  inch  in  diam- 
eter, but  in  a  young  ovary 
it  is  a  microscopic  cell. 
The  "  white  "  or  albumen 
which  surrounds  the  yolk 
and  also  the  shell  are  se- 
creted around  the  egg  as 
it  passes  through  the  ovi- 
duct on  the  way  to  the 
exterior.  Obviously,  the 
"  yolk  "  is  the  real  egg,  corresponding  to  a  frog's  egg,  and  the 
" white"  and  shell  are  later  additions  formed  like  the  jelly 
around  frog's  eggs. 


FIG.  151.  Diagram  of  bird's  egg.  yk, 
yolk  ;  alb,  white  or  albumen  ;  bl,  germ- 
disc  ;  ch,  thickened  albumen  which 
holds  yolk  in  position ;  sh.m,  two 
shell  membranes.  (From  Parker  and 
Haswell) 


448 


APPLIED  BIOLOGY 


Careful  observation  of  the  "  yolk"  of  a  bird's  egg  will  dis- 
close a  white  spot  on  its  upper  surface.  To  see  this,  lay  a 
fresh  egg  on  some  cotton  or  sawdust, 
carefully  chip  the  shell  with  forceps, 
or  cut  with  scissors,  making  a  hole 
about  one  inch  in  diameter.  This 
white  spot  is  the  germ-disc  from 
which  the  embryo  will  develop ; 
and  all  the  other  material  inside 
the  yolk-membrane  is  food.  Hen's 
eggs  kept  under  sitting-hens,  or  in 
incubators,  for  15,  24,  36,  48,  and  72 
hours,  and  then  opened  (as  directed 
above)  will  show  a  series  of  stages 
in  the  growth  of  the  embryo  from 
the  germ-disc.  Such  stages  are 
FIG.  152.  Germ-disc  with  sometimes  removed  from  eggs  and 
chick  embryo  after  about  prepared  for  microscopic  examina- 

36    hours  incubation,     hd,      .  T/.  . 

head   forming  ;    ng,  neural    tion.     If  Such  preparations  are  avail- 
groove    (first    appearance   able,  they  should  be  examined. 

of  nervous  system).    (From         ^  ,  .   ,     ,  ,  ,  , 

Marshall.)  Eggs  which  have  been  incubated 

for  several  days  show  the  embryo 

surrounded  by  membranes  in  part  of  which  there  are  blood- 
capillaries.  The 
blood  in  these  is 
pumped  by  the  em- 
bryo's heart,  which 
begins  to  beat  on  the 
second  day  of  incuba- 
tion. The  purpose  of 
circulating  blood  in 
the  blood-vessels  of 
membranes  outside 
the  embryo's  body  is  FlGA  153'  A'  chif  eKmb7?  of  ,about.f?ren 

.     .  J  days.     B,  one  of   about   ten   days  with  all 

(1)     Obtaining     food         organs  formed.     (From  Parker  and  Haswell.) 


THE  VERTEBRATES  449 

from  the  yolk  and  later  from  the  "  white/'  (2)  obtaining 
oxygen  from  air  which  filters  in  through  the  pores  of  the 
shell,  and  (3)  the  discharge  of  carbon  dioxide  made  by  the 
developing  embryo.  The  importance  of  the  two  later  pro- 
cesses, which  considered  together  constitute  the  respiration 
of  the  egg,  is  shown  by  the  fact  that  the  embryo  dies  from 
asphyxiation  (lack  of  oxygen  and  excess  of  carbon  dioxide) 
if  the  pores  of  the  shell  become  filled,  as  when  coated  with 
albumen  from  another  egg  accidentally  broken.  Hence, 
poultry  keepers  must  take  great  care  to  keep  eggs  intended 
for  hatching  clean  before  and  during  incubation. 

The  time  for  incubation  varies.  Hens'  eggs  hatch  in  three 
weeks,  ducks'  eggs  in  four  weeks,  and  other  birds  have  either 
shorter  or  longer  incubation.  No  one  knows  why  the  egg 
of  one  species  of  bird  requires  more  time  than  does  that  of 
others. 

366.  Reptile  Development.  —  The  embryology  of  many 
snakes,  alligators,  and  turtles  is  very  similar  to  that  of  birds, 
except  that  incubation  is  due  to  the  sun's  heat  ancf  not  to 
brooding  by  parents.    Some  snakes  and  lizards  are  viviparous, 
the  eggs  being  retained  in  the  oviducts  until  fully  developed 
into  young  animals  which  are  then  ejected  by  muscular  con- 
traction of  the  oviducts.      The  eggs  of    these   viviparous 
reptiles  are  similar  to  the  oviparous  ones  and  contain  much 
stored  food  to  be  used  by  the  developing  embryos. 

367.  Mammalian  Development.  —  We  have  already   (in 
§  63)  noted  the  advantages  of  internal  or  viviparous  develop- 
ment over  the  external  or  oviparous  method ;  and  that  while 
cases  of  internal  development  occur  among  animals  of  many 
groups,  it  is  in  the  mammals  that  we  find  vivipary  universal, 
with  the  one  exception  of  the  Australian  duck-bill,  which 
lays  eggs. 

In  adaptation  to  viviparous  development  the  oviducts  of 
the  mammals  have  been  modified  into  a  sac-like  structure  in 
whose  cavity  eggs  lie  during  their  embryonic  development. 

2G 


450  APPLIED  BIOLOGY 

Such  a  cavity  adapted  to  holding  embryos  is  called  a  uterus 
or  womb.  In  some  low  mammals,  like  kangaroos  and  opos- 
sums, there  is  a  right  and  a  left  uterus  formed  by  expansion 
of  part  of  each  oviduct.  These  lie  in  the  same  position  as  the 
oviducts  of  frogs.  In  higher  mammals  the  right  and  left  uteri 
grow  together  during  embryonic  life,  and  so  there  is  a  single 
uterus  with  a  tube  (Fallopian)  extending  to  each  ovary  (right 
and  left).  Egg-cells  formed  and  discharged  by  either  ovary 
pass  through  the  tube  into  the  uterus  and  there  develop 
into  embryos. 

In  all  mammals  the  egg-cells  discharged  are  fertilized  near 
the  ovaries  in  the  Fallopian  tubes  by  sperm-cells  which  have 
arrived  there  by  swimming  through  the  secretions  on  the  liv- 
ing membranes  of  the  uterus  and  tubes.  The  fertilized  egg 
begins  to  divide  at  once  and  may  have  undergone  con- 
siderable development  when,  after  a  few  days,  it  slips  from 
the  tube  into  the  uterus. 

The  number  of  egg-cells  fertilized  at  one  time  varies  in 
different  species.  It  is  well  known  that  many  domesticated 
animals  (e.g.,  sheep,  cow,  horse)  usually  have  one  offspring 
at  a  time;  but  some  occasionally  produce  two  (twins)  or 
even  three  (triplets).  Others  commonly  produce  many 
young  at  a  time  (e.g.,  dog,  cat,  pig,  rabbit,  mice).  The  num- 
ber of  young  produced  indicates  the  number  of  egg-cells 
which  were  matured  and  fertilized. 

The  period  of  development  in  the  uterus  from  fertilization 
to  birth  of  the  young  is  commonly  known  as  gestation  or 
pregnancy;  and  the  length  of  time  is  highly  variable.  It  is 
approximately  21  days  in  guinea-pig,  30  days  in  rabbit  and 
squirrel,  55  days  in  cat,  62  days  in  dog,  3  months  in  lion, 
4  months  in  pig,  5  months  in  sheep  and  goat,  6  months  in 
bear,  9  months  in  cow,  over  9  months  (280  days)  in  human 
species,  10  months  in  whale,  11  months  in  horse,  14  months 
in  giraffe,  and  22  months  in  elephant.  These  are  simply 
illustrations  selected  from  familiar  mammals. 


THE    VERTEBRATES 


451 


In  order  to  provide  for  the  nutrition  and  respiration  of 
embryo  mammals,  a  complicated  connection  is  made  be- 
tween the  blood-system  of  the  embryo  and  that  of  the  mother. 
Figure  154  shows  a  rabbit  embryo  with  its  surrounding  mem- 
branes. These  are  abundantly  supplied  with  blood-vessels 
connected  with  the  embryo's  heart.  Figure  155  shows  the 
position  of  an  embryo  mammal  in  a  uterus.  The  darkly 
shaded  area  around  the  embryo  represents  lining  tissue 
(epithelium)  of  the  uterus,  and  this  tissue  receives  its  blood- 
supply  from  the  heart  and  arteries  of  the  mother.  The  tree- 
like processes 
shown  in  Fig. 
155  are  further 
outgrowths  of 
the  irregular 
processes  shown 
(in  black)  on 
the  outer  mem- 
branes of  the 
embryo  repre- 
sented in  Fig. 
154.  These  tree- 
like processes  re- 
ceive their  blood- 
supply  from  the 
embryo's  heart 
through  blood- 
vessels in  the 


FIG.  154.  Rabbit  embryo  of  twelve  days'  develop- 
ment, with  its  surrounding  membranes,  which  serve 
for  attachment  to  the  lining  of  the  uterus.  (From 
Marshall) 


umbilical  cord  attached  to  the  embryo  at  the  umbilicus  or 
navel.  As  a  result  of  this  close  attachment  of  the  mem- 
branes of  the  embryo  and  the  lining  of  the  uterus,  the  blood- 
vessels of  the  two  are  near  enough  to  allow  osmosis.  From 
the  maternal  blood-capillaries  foods  and  oxygen  osmose  into 
those  of  the  embryo,  and  excretions  of  the  embryo's  cells 
pass  into  the  maternal  blood.  Solid  bodies,  like  red  blood- 


452 


APPLIED  BIOLOGY 


cells,  cannot  pass  from  the  maternal  to  the  embryo's  blood; 
and  the  blood-cells  in  the  embryonic  blood-vessels  are 
formed  from  certain  cells  belonging  to  the  embryo.  How- 
ever, the  important  point  is  that  food,  oxygen,  and  excre- 
tions osmose  between  the 
maternal  and  embryonic 
blood-vessels  in  the  mem- 
branes which  attach  the 
embryo  to  the  wall  of  the 
uterus. 

The  membranes  which 
attach  the  embryo  to  the 
uterus  constitute  the  pla- 
centa. It  normally  separates 
from  the  uterus  after  the 
birth  of  an  embryo,  and  is 
then  itself  discharged  by 
muscular  contractions  of 
the  uterus. 

The  fact  that  an  embryo 
attaches  to  the  wall  of  the 
uterus  as  above  described 
and  is  thus  enabled  to  get 
food  explains  the  large  size 
of  many  mammals  at  birth. 
The  eggs  of  all  mammals 
are  transparent  cells  of  mi- 
croscopic size,  and  embry- 
onic growth  is  due  to  food 
supplied  by  the  maternal 
blood-vessels. 

It  should  be  noted  that  the  entire  time  of  gestation  is 
not  occupied  with  the  formation  of  the  embryo's  organs. 
For  example,  an  embryo  may  develop  in  two  or  three  months 
so  that  it  has  the  form  and  structure  of  an  adult,  but  may  be 


FIG.  155.  Diagram  of  a  mammalian 
uterus  showing  attachment  of  an  em- 
bryo to  the  lining  (black  in  the  fig- 
ure). The  umbilical  cord,  from  the 
ventral  surface  of  the  embryo's  abdo- 
men, extends  to  the  tree-like  pro- 
cesses embedded  in  the  lining  of  the 
uterus.  The  black  lines  in  the  um- 
bilical cord  indicate  arteries  and 
veins  connected  with  the  embryo's 
heart.  The  openings  of  the  uterus 
shown  at  upper  right  and  left  are  to 
the  Fallopian  tubes  leading  to  the 
ovaries,  while  the  lower  opening  is 
the  mouth  of  the  uterus  through 
which  the  mature  embryo  is  finally 
expelled  by  muscular  contraction. 
(From  Marshall.) 


THE  VERTEBRATES  453 

held  in  the  uterus  many  more  months  in  order  to  afford 
protection  and  nourishment  while  it  grows  larger  and  stronger. 
This  later  stage  after  the  organs  are  formed  is  often  called 
a  foetus,  so  as  to  reserve  the  word  embryo  for  the  early  stages 
when  the  egg  is  forming  organs. 

In  the  limited  time  available  in  this  course  we  cannot  do 
more  than  study  the  mere  outlines  of  mammalian  develop- 
ment as  stated  in  the  foregoing.  There  are  many  facts  in 
this  line  which  are  especially  interesting  because  of  the  light 
which  they  throw  on  human  life,  and  for  these  the  reader 
must  be  referred  to  special  books,  and  to  college  courses  in 
embryology  of  animals. 


PART   IV 

PRINCIPLES    OF    BIOLOGY    APPLIED    TO 
HUMAN  STRUCTURE  AND    LIFE 

368.  Human  Biology.  —  Biology  is  the  science  of  living 
things,  and  human  biology  may  be  defined  as  the  study  of 
man  considered  as  a  living  thing  and  interpreted  in  the  light 
of  studies  of  other  living  things. 

The  justification  for  including  study  of  man  as  part  of 
biology  is  found  in  the  fact  that  the  human  body  in  its 
structure  and  functions  is  remarkably  like  animals,  the 
higher  forms  in  particular.  In  the  body  of  man  are  the  same 
organs  as  in  the  animals  known  as  beasts  or  mammals; 
and  the  organs  of  man  and  the  beasts  are  closely  alike 
even  in  microscopic  details.  This  similarity  also  appears 
when  comparing  man  with  still  lower  animals.  In  short, 
when  biologists  consider  the  close  similarity  of  structure  and 
function  in  man  and  various  types  of  animals,  they  see  no 
escape  from  the  conclusion  that  man's  relation  to  the  animal 
kingdom  is  as  stated  in  the  next  paragraph. 

369.  The  Classification  of  Man.  —  (1)  As  suggested  above, 
man  belongs  to  the  animal  kingdom,  because  his  body  is 
built   on  the   plan  of  structure  found  in  many  animals. 
(2)    Man  is  a  backboned  or  vertebrate  animal,  because  he  pos- 
sesses a  backbone  or  vertebral  column.     (3)  Man  belongs  to 
the  class  of  mammals  or  Mammalia,because  he  has  the  three 
characteristics  of  this  group,  —  hair,  diaphragm,  and  mam- 
mary glands.     (4)  Man  belongs  to  the  order  of  the  Primates, 
because  his  body  is  in  numerous  respects  more  similar  to  apes 

455 


456  APPLIED  BIOLOGY 

than  to  other  animals.  (5)  Man  belongs  to  the  human  family, 
the  genus  Homo,  and  the  one  species  sapiens  (a  word  meaning 
wise,  and  referring  to  the  fact  that  man's  intellectual  develop- 
ment is  characteristic  and  markedly  distinguishes  the  human 
species  from  all  other  animals).  In  fact,  it  is  in  the  highly 
developed  functioning  of  the  nervous  system  alone  that  man 
stands  distinctly  differentiated  from  the  highest  apes  and 
other  animals.  This,  however,  is  the  proper  field  of  the 
science  of  psychology  (the  study  of  mind  or  mental  phenom- 
ena), which  should  be  taken  up  in  college  or  in  private  read- 
ing after  graduation  from  high  school.  So  far  as  biology  is 
directly  concerned,  it  simply  has  to  take  into  consideration 
the  demonstrated  fact  that  man  and  other  animals  are  re- 
markably similar  in  structure  and  functions.  This  similarity 
is  fortunate,  for  it  makes  possible  the  application  to  human 
biology  of  many  facts  which  were  first  learned  by  the  study  of 
various  animals.  (6)  Finally,  the  human  species  has  five 
varieties  or  races,  —  Caucasian,  American  Indian,  Mongolian, 
Malay,  Ethiopian, — each  with  certain  peculiarities.  (Look 
up  these  races  in  any  advanced  textbook  of  geography,  or  in 
an  unabridged  dictionary,  and  report  briefly  concerning 
their  characteristics  and  geographical  distribution.) 

Study  of  the  customs,  character,  history,  and  institutions 
of  races  of  men  is  the  science  of  ethnology.  The  science  deal- 
ing with  the  laws  of  human  society  is  sociology.  General 
study  of  man,  combining  facts  of  ethnology,  biology,  psy- 
chology, and  sociology,  is  anthropology.  The  following  brief 
definitions  in  parentheses  will  help  the  memory  :  psychology 
(science  of  mind) ;  sociology  (science  of  society) ;  ethnology 
(science  of  human  races) ;  anthropology  (science  of  man). 
Each  of  these  lines  of  study  of  human  life  is  now  so  highly 
developed  that  special  books  are  necessary;  but  they  all 
are  founded  on  biology  to  such  an  extent  that  a  knowledge 
of  that  science  is  important  for  the  reader  of  any  of  the 
special  sciences  dealing  with  man. 


CHAPTER  XVII 

HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES 

370.  General   Plan   of    Human   Body.  —  Like   the   frog, 
already  studied,  the  human  body  consists  of  head,  trunk, 
and  limbs.     The  trunk  is  composed  of  the  chest  or  thorax, 
and  the  belly  or  abdomen.     The  arms  are  the  upper  or  an- 
terior limbs ;  the  legs  are  the  lower  or  posterior  limbs.     Note 
that  the  thigh  of  a  leg  corresponds  in  structure  to  the  upper 
arm,  the  shank  to  the  forearm,  the  ankle  to  the  wrist,  the 
toes  to  the  fingers.     Note  also  that  externally  the  human 
body  is  bilaterally  symmetrical. 

371.  Skeleton.  —  Like  the  frog  and  all  the  other  back- 
boned animals,  the  human  body  is  supported  by  an  internal 
framework  or  skeleton  composed  of  bone  and  cartilage,  which 
is  chiefly    at  the  ends  of  bones.     There  are  more  than  200 
bones  in  the  human  skeleton ;   33  vertebrae  or  segments  of 
the  spinal  column,  about  25  bones  in  the  skull  of  an  adult, 
24  ribs,  30  bones  in  each  arm  and  leg,  and  the  bones  in  the 
shoulder-girdle   and   pelvis.     The  number   of  bones  varies 
with  the  age ;   for  example,  the  skull  bones  are  more  numer- 
ous in  young  children,  but  they  grow  or  fuse  together  as  the 
individual  becomes  older.     Each  half  of  the  pelvis  is  com- 
posed of  three  bones  which  have  fused  together.     Nine  bones 
(vertebrae)  at  the  posterior  end  of  the  spinal  column  are 
fused  together  in  connection  with  the  pelvis,   leaving   24 

separate  vertebrae. 

457 


458  APPLIED  BIOLOGY 

(L)  The  best  way  to  study  the  human  skeleton  is  to  compare  a 
mounted  skeleton  with  labeled  drawings  in  textbooks  of  anatomy 
and  physiology.  At  the  same  time  the  pupil  should  locate  the  posi- 
tion of  the  larger  bones  in  his  own  body.  If  a  mounted  skeleton  is 
not  owned  by  the  school,  the  pupil  should  locate  as  nearly  as  possible 
the  bones  in  his  own  body,  using  labeled  pictures  as  a  guide.  The 
names  of  the  large  bones  are  so  frequently  referred  to  that  it  is  desir- 
able to  memorize  them.  Pupils  who  are  studying  drawing  will 
find  various  parts  of  the  skeleton  good  objects  for  sketching ;  but 
as  a  rule  this  work  is  not  possible  in  the  limited  time  available  for  the 
biology  class-work. 

The  most  important  parts  to  notice  while  examining  a  skeleton 
are :  (1)  The  backbone  or  vertebral  column,  to  which  all  other  parts 
of  the  skeleton  are  attached.  It  is  the  central  supporting  axis  of 
the  body.  (2)  The  bones  of  the  two  pairs  of  limbs,  comparing  the 
anterior  with  the  posterior  pairs.  (3)  The  ribs  and  the  bones  which 
connect  the  arms  with  the  backbone.  (4)  The  pelvis,  which  con- 
nects the  legs  with  the  backbone.  (5)  The  larger  bones  of  the  skull. 

372.  Body-wall  and  Body-cavity.  —  As  in  the  frog,  the 
outer,  fleshy  wall  which  incloses  the  internal  organs  is  the 
body-wall;  the  internal  cavity  is  the  body-cavity.  In  the  frog 
there  is  one  cavity  in  which  lie  the  heart,  lungs,  liver, 
stomach,  intestine,  kidneys,  and  reproductive  organs.  In 
the  human  body  the  diaphragm  forms  a  partition  across 
the  body-cavity,  dividing  it  into  the  anterior  (upper)  cavity 
containing  the  heart  and  lungs  and  known  as  the  thoracic 
cavity  (chest-cavity),  and  the  posterior  (lower)  abdominal 
cavity,  which  contains  all  the  internal  organs  except  the 
heart  and  lungs.  The  thoracic  cavity  is  inclosed  by  the 
ribs,  while  the  abdominal  cavity  is  bounded  by  the  muscular 
walls  of  the  abdomen. 

Structure  of  the  Body-wall.  —  (D)  This  is  essentially  the  same  in 
man  and  other  mammals,  and  so  we  may  study  any  of  the  ani- 
mals found  in  meat-markets.  A  slice  of  bacon  will  serve  our  pur- 
pose. On  the  one  edge  of  the  slice  is  the  skin  or  "rind."  This, 
of  course,  was  the  outside  skin  of  the  pig.  The  streaks  of  lean 
meat  are  muscles  of  the  body-wall.  Fat  has  been  deposited  between 
the  muscles,  and  also  between  the  muscles  and  the  skin.  The  thick- 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       459 

ness  of  the  body-wall  depends  upon  the  amount  of  fat ;  hence  very 
lean  bacon  is  relatively  thin.  Notice  that  the  skin  is  fastened 
firmly  to  the  muscles  and  fat ;  this  is  due  to  fibers  of  connective 
tissue.  In  some  parts  of  the  body  of  mammals  the  skin  is  not 
fastened  down  so  closely  as  in  bacon ;  e.g.,  note  the  loose  skin  on 
the  back  of  your  own  hand,  or  on  the  backs  of  young  puppies. 

373.  Relative  Positions  of    Internal    Organs  in   Man.  — 

Study  diagrams,  charts,  and  a  manikin,  if  available ;  and  note 
the  following  positions  of  the  largest  organs,  which  are  in  the 
same  relative  position  as  in  the  frog.  (1)  The  body-cavity 
is  ventral  to  the  backbone.  (2)  The  alimentary  canal  ex- 
tends through  the  body-cavity  from  anterior  to  posterior. 
(3)  The  heart  is  ventral  to  the  alimentary  canal  (esophagus 
part).  (4)  The  liver  lies  ventral  to  the  alimentary  canal 
(stomach  and  intestine  part).  (5)  The  kidneys  lie  dorsal  to 
the  alimentary  canal  and  in  the  posterior  part  of  the  body- 
cavity  (abdominal  part).  (6)  The  brain  and  spinal  cord  lie 
in  the  cavities  formed  by  the  bony  skull  and  the  backbone. 

374.  Life-activities  of  the  Human  Body.  —  The  character- 
istics of  living  things  already  studied  in  connection  with 
animals  and  plants  apply  to  the  human  body,  for  it  is  a 
living  mechanism  which  performs  the  functions  necessary 
for  life. 

The  life-activities  are  located  in  the  cells;  and  these, 
as  in  the  case  of  the  frog,  are  in  the  tissues  (epithelial, 
connective,  muscular,  bony  or  osseous,  cartilaginous,  and 
nervous) . 

Probably  the  two  activities  which  most  attract  our  atten- 
tion are  those  concerned  with  food  and  breathing.  Moreover, 
these  are  the  basis  of  all  the  other  processes  occurring  in  the 
human  body.  For  these  reasons  we  shall  specially  consider 
the  taking  of  foods  and  oxygen  into  the  human  body,  and 
later  we  shall  trace  the  changes  of  foods  and  oxygen  in  the 
body.  This  will  lead  us  on  to  consider  all  the  essential  life- 
activities. 


460  APPLIED  BIOLOGY 

FOODS 

375.  What  are  Foods?  —  A  convenient  definition  for  our 
present  purposes  is  that  foods  are  solid  or  liquid  substances 
which  when  taken  into  the  alimentary  canal  are  useful  in  the 
life-processes  of  our  bodies.     In  most  cases  substances  able 
to  serve  as  our  foods  must  be  capable  of  being  digested  and 
absorbed  as  materials  for  energy,  repair,  and  growth ;   but 
a  certain  amount  of  plant  cell-walls  is  useful,  although  not 
digestible,  in  the  human  alimentary  canal. 

Sources  and  Kinds  of  Human  Foods.  —  (L)  Write  in  your  note- 
book a  list  of  some  common  foods,  arranging  in  three  columns  those 
of  animal,  plant,  and  mineral  origin. 

376.  Nutrients.  —  We  might  consult  cook-books  and  make 
a  very  long  list  of  the  names  of  prepared  foods  which  are 
served  on  our  tables ;  but  these  are  made  by  combinations  of 
such  common  ingredients  as  meats,  vegetables,  milk,  butter, 
lard,    sugar,   flour,   starch,   chocolate,   salt,   etc.     Chemists 
have  shown  that  these  common  things  which  are  used  in 
every  kitchen  in  combining  our  foods  are  composed  of  cer- 
tain chemical  compounds  known  as  sugar,  starch,  fat  (oil), 
proteins,  and  minerals.     These  compounds,  from  which  all 
the  combinations  of  human  foods  are  made,  are  known  in 
physiology  as  nutrients. 

In  order  to  prove  that  various  foods  are  made  up  of  these 
few  nutrients,  we  need  to  know  some  method  of  identifying 
each.  Fortunately  chemists  have  discovered  some  simple 
tests  which  are  easily  applied,  as  directed  below. 

377.  Chemical  Tests  for  Nutrients.  —  Certain  chemicals  produce 
characteristic  reactions  on  nutrients  and  hence  may  be  used  to  detect 
their  presence  in  mixtures  of  foods. 

Starch  Test.  —  (D  or  L)  Repeat  the  iodine  test  with  diluted 
starch  paste  (§  100). 

Sugar  Test.  —  (D  or  L)  Boil  a  few  grapes,  raisins,  or  prunes  in 
water  in  a  test-tube  for  two  minutes,  and  this  will  extract  some  sugar. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       461 

Into  another  test-tube. put  some  Fehling's  reagent,*  the  ingredients 
of  which  are  usually  kept  in  separate  bottles  and  mixed  when  needed. 
Heat  the  Fehling's  reagent,  and  then  with  a  pipette  slowly  drop 
into  it  some  of  the  sugar  solution  obtained  from  the  dry  fruits  ;  or 
drop  some  of  the  reagent  into  the  sugar  solution.  A  red  color  indi- 
cates the  presence  of  one  kind  of  sugar.  One  such  experiment 
could  not  prove  that  when  Fehling's  reagent  causes  a  red  color  it 
means  that  sugar  is  present,  for  we  have  not  yet  tried  this  solution 
on  starch,  fat,  and  other  things.  However,  chemists  have  tried  the 
Fehling's  reagent  on  all  the  substances  commonly  found  in  animals 
and  plants,  and  it  has  been  demonstrated  that  only  certain  kinds  of 
sugars  produce  the  red  color.  We  have  not  time  to  repeat  such 
inves  igations,  and  so  we  must  accept  the  chemists'  statement  that 
Fehling's  reagent  is  a  test  for  a  sugar  found  in  grapes  and  known  as 
grape-sugar  or  dextrose.  The  same  sugar  in  corn-syrup  is  popularly 
called  glucose.  Its  chemical  formula  is  CeH^Oe.  Ordinary  sugars 
sold  in  stores  are  chiefly  cane-sugar  (CizHaaOn) .  Milk-sugar  (lactose) 
and  malt-sugar  (maltose)  have  the  same  chemical  formulas  as  cane- 
sugar  (sucrose). 

Kinds  of  Sugars.  —  (D  or  L)  Dissolve  some  commercial  glucose, 
or  corn-syrup,  in  water  and  test  with  Fehling's  reagent.  Dissolve 
some  ordinary  "granulated"  sugar  in  water,  and  test.  This  latter 
will  sometimes  give  no  red  color  until  after  it  has  been  boiled  for  some 
time  or  treated  with  strong  acids.  The  explanation  is  that  most 
"granulated"  sugar  is  chiefly  the  kind  known  as  cane-sugar,  while 
sugars  which  give  red  color  in  Fehling's  test  are  of  the  kinds  known  as 
grape-sugar  and  fruit-sugar.  The  boiling  or  treating  with  acid 
changes  the  cane-sugar  into  the  other  kinds ;  and  these  can  then  be 
tested  with  Fehling's  reagent.  Taste  granulated  sugar  and  glucose, 
and  compare  as  to  sweetness.  As  a  practical  point,  it  is  interesting 
to  note  that  granulated  sugar  should  be  added  to  fruits  after  cooking. 
If  added  before,  the  sweetness  will  be  partly  lost  by  change  to  the 
other  sugars,  which  are  as  good  food,  but  less  sweet. 

Tests  for  Fat.  —  (D  or  L)  Put  a  drop  of  olive  oil  on  a  sheet  of  white 
paper,  and  note  that  a  grease-spot  is  produced.  Dissolve  some  beef- 
suet  in  a  small  quantity  of  benzine  or  ether  (keep  such  volatile 
liquids  as  these  far  away  from  a  flame),  put  a  drop  of  the  solution  on 
paper,  and  notice  the  spot  left  after  evaporation  of  the  benzine. 

*  Fehling's  reagent,  a  test  for  certain  kinds  of  sugar,  may  be  purchased 
from  dealers  in  chemicals  or  at  ordinary  drug-stores.  It  may  be  made  ac- 
cording to  the  formula  in  the  "Teachers'  Manual"  which  accompanies  this 
book. 


462  APPLIED  BIOLOGY 

Or  lay  a  piece  of  suet  on  a  paper  and  heat  slowly.  This  "grease- 
spot"  test  is  a  simple  way  of  finding  whether  fats  or  oils  are  present 
in  foods. 

Tests  for  Protein.  —  (D  or  L)  Mix  a  small  quantity  of  white-of-egg 
in  water  in  a  test-tube,  shake  well,  add  some  strong  nitric  acid, 
boil  until  the  solution  turns  yellow,  then  add  drops  of  ammonia 
until  an  orange  color  appears.  Or  instead  of  the  acid,  add  to  the 
egg-albumen  in  water  some  drops  of  Millon's  reagent  (mercury 
dissolved  in  nitric  acid ;  obtainable  from  chemists),  heat  slowly,  and 
red  color  will  appear,  especially  after  cooling. 

Tests  for  Water  and  Minerals.  —  The  loss  of  weight  by  drying  foods 
is  chiefly  due  to  evaporation  of  water.  The  ashes  left  after  burning 
foods  represent  the  mineral  contents.  By  complicated  processes 
chemists  can  analyze  the  ashes  and  determine  the  kind  and  propor- 
tion of  elements  present. 

378.  Sugars  and  Starches  :  Carbohydrates.  —  All  sugars 
and  starches  are  grouped  together  under  the  name  of  car- 
bohydrates.  They  contain  but  three  elements :  carbon, 
hydrogen,  and  oxygen.  We  have  already  learned  that  starch 
is  formed  in  plant  cells  which  have  chlorophyll,  and  that 
starch  is  readily  digested  by  enzymes  into  sugar,  or  sugar 
turned  back  again  to  starch  for  storage.  The  carbohydrates 
which  are  used  as  human  foods  are  chiefly  cane-sugar  or 
sucrose  and  milk-sugar  or  lactose  (both  with  the  formula 
Ci2H220n) ;  grape-sugar  and  fruit-sugar  (both  with  the 
formula  CeHiaOe) ;  and  starch  (formula  is  some  multiple  of 
C6H1005). 

As  we  have  seen,  grape-sugar  is  found  in  raisins  and  other 
fruits.  Under  the  names  of  glucose  and  corn-syrup  it  is  com- 
mon in  the  markets,  and  is  made  by  treating  the  starch  of 
corn  grains  with  strong  sulphuric  acid.  Lactose  or  milk- 
sugar  is  sold  in  all  drug-stores  for  use  in  preparing  foods  for 
infants  and  invalids.  Malt-  or  barley-sugar  (maltose)  is 
also  sold,  especially  for  flavoring  candies,  etc.  The  common 
"  granulated  "  sugars  in  our  markets  are  sucrose  from  the 
juices  of  sugar-cane  and  sugar-beet.  The  yellow  and  brown 
sugars  are  the  crude  sugars  obtained  by  evaporating  the  juice 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       463 

pressed  from  cane  and  beet ;  and  by  refining  processes,  these 
dark-colored  sugars  are  made  into  the  "  soft  white  "  and 
"  granulated  "  sugars.  Maple  sugar,  from  the  sap  of  maple 
trees,  is  chemically  the  same  as  cane-sugar,  but  flavored 
with  peculiar  substances  found  in  the  maple  sap. 

379.  Fats.  —  This  includes  all  kinds  of  fats  and  oils  from 
animals  and  plants.     Common  examples  are  butter,  lard, 
beef -suet  or  tallow,  olive  oil,  cotton-seed  oil,  fat  of  meats,  oil 
of  nuts.     Like  the  sugars  and  starches  (carbohydrates),  fats 
contain  only  three  elements  :  carbon,  hydrogen,  and  oxygen. 
The  sugars,  starches,  and  fats  taken  together  are  often  called 
"  non-nitrogenous  foods,"  because  they  have  no  nitrogen. 

380.  Proteins.  —  Formerly   spelled   "  proteids."     White- 
of-egg   ("  egg-albumen  "),  lean  meat,  and  milk  curds  have 
abundant  proteins.     They  contain  carbon,  hydrogen,  nitro- 
gen, oxygen,  and  sulphur.     Also,  some  have  phosphorus  and 
iron.     It  appears  then  that  all  proteins  differ  from  carbo- 
hydrates and  fats  in  having  nitrogen  and  sulphur.     As  already 
stated,  only  plants  can  make  proteins,  and  animals  must  get 
them  directly  or  indirectly  from  plants. 

381.  Albuminoids.  —  This  term,  which  means  albumen- 
like,  is  applied  to  certain  food  substances  which  have  the 
composition  of  proteins  (formerly  called  albumens) .     Gelatin 
is  the  most  common  example.     The  finer  quality  used  for 
human  food,  for  photographic  plates,  and  for  bacteriological 
study  is  obtained  by  cooking  connective  tissues,  tendons,  and 
marrow  of  bones.     Pigs'   feet,  which  are   by  many  people 
considered  a  delicacy,  produce  much  gelatin  when  boiled. 
Common  furniture  glue  is  a  crude  gelatin  from  the  connective 
tissue  of  the  hides  and  hoofs  of  animals.     Fish  glue  is  a  gelatin 
made  from  the  delicate  membranes  of  the  air-bladders  of  fishes. 
Common  gelatin  is  the  only  albuminoid  used  extensively  for 
human  food.     Although  having  the  composition  of  proteins, 
gelatin  cannot  take  their  place  as  food.     Dogs  have  been 
found  to  live  well  with  only  protein  foods  (e.g.,  lean  meat); 


464 


APPLIED  BIOLOGY 


but  with  gelatin  alone  they  soon  begin  to  lose  weight  and 
to  show  other  evidences  of  starvation.  Hence,  gelatin  must 
be  used  with  protein  foods. 

382.  Inorganic  or  Mineral  Foods,  and  Water.  —  The  in- 
organic foods  are  the  only  ones  not  formed  by  animals  or 
plants.     As  we  have  seen,  water  plays  an  important  part  in  all 
living  matter.     In  the  human  body  it  is  especially  important 
in  dissolving  foods  during  digestion,  and  also  as  the  circulating 
medium  in  the  blood-  and  lymph-systems. 

Common  salt  (sodium  chloride,  NaCl)  is  only  one  of  a  num- 
ber of  mineral  salts  necessary  in  the  human  body.  A  com- 
pound containing  iron  gives  the  red  color  to  blood ;  lime 
(calcium)  is  necessary  in  the  bones;  and  less  noticeable 
quantities  of  other  elements  (P,  K,  S,  Mg)  are  needed  in  the 
human  body.  Most  animal  and  vegetable  foods  which  we 
commonly  use  contain  these  necessary  elements,  and  so  we 
do  not  have  to  give  any  special  attention  to  obtaining  them. 
Common  salt  is  the  only  mineral  food  which  is  regularly  added 
to  our  diet  in  addition  to  what  is  naturally  in  our  organic 
foods;  and  it  is  probably  true  that  we  use  this  greatly  in 
excess  of  what  the  body  actually  requires. 

383.  Testing  Foods  for  the  Nutrients.  —  (D  or  L)    Apply  the  tests 
for  starch,  proteins,  and  grape-sugar  to  oatmeal,  flour,  white-of-egg, 
egg-yolk,  potato,  onion,  rice,  beans,  peas,  lean  meat,  apple,  honey, 
corn-syrup,  pears,  corn-meal,  and  other  common  articles  of   food. 
Place  each  food  to  be  tested  in  some  water  in  a  test-tube,  boil  for  a 
few  minutes,  and  then  pour  in   the   testing  reagent  to  be  used. 
Make  a  table  in  your  note-book  and  record  the  results  of  the  tests 
by  making  a  mark  in  the  proper  columns  for  the  nutrient  found  to 
be  present. 


NAME  OP  FOOD  TESTED 

CONTAINS 
SUGAR 

CONTAINS 
STARCH 

CONTAINS 
PROTEIN 

CONTAINS 
FAT 

HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       465 

384.  Foods  are  Combinations  of  the   Nutrients.  —  From 
the  foregoing  experiments,  it  is  evident  that  one  or  more 
of  the  nutrients  is  present  in  each  of  the  common  foods. 
Chemists  have  proved  that  the  useful  constituents  of  all  our 
foods  are  carbohydrates,  fats,  proteins,  mineral  salts,  and 
water.     The    above    experiments    also    showed    that    some 
foods  have  a  preponderance  of  certain  nutrients;  e.g.,  white- 
of-egg  is  chiefly  protein,  potato  is  chiefly  starch,  corn-syrup 
is  almost  pure  sugar,  butter  and  beef-suet  are  largely  fat.     It 
is  evident  that  if  all  the  nutrients  are  needed  in  human  diet, 
they  can  best  be  obtained  by  a  mixture  of  foods ;  that  is,  a 
meal  composed  of  lean  meat,  potato,  bread,  butter,  and  some 
form  of  sugar  might  be  arranged  to  supply  equal  amounts  of 
protein,  fat,  sugar,  and  starch.     We  shall  see  later  that  equal 
amounts  of  these  nutrients  are  not  needed;    but  that  a 
mixture  of  foods  is  necessary  in  order  to  give  the  proper 
amount  of  each  of  the  nutrients  of  which  our  common  articles 
of  food  are  composed. 

STRUCTURE   OF  HUMAN  DIGESTIVE   ORGANS 

In  order  to  understand  many  points  concerning  the  work 
of  the  organs  which  deal  with  foods,  we  must  first  get  a  clear 
idea  of  the  general  structure  of  the  alimentary  canal  and  of 
its  attached  organs  which  secrete  digestive  fluids  (liver, 
pancreas,  salivary  glands).  Therefore,  we  must  for  a  time 
turn  aside  from  considerations  of  function  and  study  the 
structure  of  these  organs. 

385.  The  Mouth-Cavity.  —  (L)    Turn  your  back  to  a  window  or 
a  lamp,  and  with  a  hand-mirror  reflect  the  light  into  your  open 
mouth.     Notice  the  hard  palate  forming  the  roof  of  the  mouth- 
cavity.     At  the  back  of  the  mouth-cavity  is  the  soft  palate,  which 
separates  the  mouth-cavity  from   the   post-nasal  cavity;   and   this 
cavity  in  turn  communicates  with  the  cavities  of  the  nose.     Take 
"short  breaths"  and  notice  the  effect  upon  the  soft  palate.     Apply 
your  tongue  to  the  roof  of  the  mouth  and  slowly  move  it  backward 

2n 


466  APPLIED  BIOLOGY 

and  forward  until  you  feel  the  shape,  position,  and  texture  of  the 
hard  and  soft  palates.  Press  down  upon  the  tongue  with  a  clean 
(sterile)  glass  rod,  or  the  handle  of  a  spoon,  and  examine  the  small 
prolongation  of  the  soft  palate  which  touches  the  tongue  when  that 
is  not  depressed.  This  is  the  uvula. 

386.  The  Teeth.  —  (L)  Examine  your  teeth,  again  using  the  hand- 
mirror,  taking  the  following  description  as  a  guide  :  Beginning  at  the 
middle  line  at  the  front  of  each  jaw,  there  are  in  order  the  following 
kinds  of  teeth  in  half  of  either  the  upper  or  the  lower  jaw :  First,  two 
chisel-shape  cutting  teeth  (incisors,  meaning  to  cut  into).  Next,  a 
tooth  with  a  more  pointed  edge,  which  corresponds  to  the  great  fangs 
of  dogs  and  cats  and  other  animals  which  must  hold  their  prey ;  hence 
the  name  canine  or  dog-teeth.  The  tusks  of  boars  and  walruses  are 
enormously  enlarged  canine  teeth.  Elephants'  tusks  are  upper  in- 
cisors. Next  back  of  the  canine  tooth  on  each  side  there  are  an  the 
first  or  "milk-set"  of  teeth  two  grinding  teeth  (molars).  This  makes 
a  total  of  twenty  teeth  in  the  first  or  milk-set,  which  are  deciduous. 
In  adults  there  are  in  each  half  of  a  jaw  two  teeth  called  bicuspids 
(meaning  two  cusps  or  points)  in  place  of  the  two  molars  of  child- 
hood; and  back  of  these  are  three  molars,  often  called  "wisdom 
teeth."  There  are,  therefore,  twelve  molars  in  adults  in  addition 
to  teeth  in  the  places  occupied  by  the  twenty  teeth  of  the  first  or 
deciduous  set,  making  a  total  of  thirty-two  for  the  adult. 

The  incisor  teeth  begin  to  appear  in  children  at  six  or 
eight  months  of  age,  and  the  full  milk-set  is  present  after 
eighteen  to  twenty-four  months.  The  loss  or  shedding  of 
these,  caused  by  growth  of  new  teeth  below,  occurs  at  various 
times  between  seven  and  twelve  years  of  age.  The  perma- 
nent teeth  begin  with  the  incisors  at  seven  or  eight  years 
and  are  completed  with  the  appearance  of  the  molars  or 
wisdom  teeth  at  between  sixteen  and  twenty  years  of  age. 
The  growth  of  the  teeth  through  the  fleshy  tissue  (gums) 
is  often  called  "  cutting  teeth/' 

The  structure  may  be  studied  by  breaking  open  an  ex- 
tracted tooth,  or  better  by  studying  a  thin  section  prepared  for 
microscopic  use.  There  is  a  central  cavity  which,  during  the 
life  of  the  tooth,  is  filled  with  a  soft  mass  composed  of  con- 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       467 

nective  tissue,  blood-vessels,  and  nerves.  This  is  the  so- 
called  pulp.  The  pulp-cavity  extends  down  into  each  of 
the  roots  of  the  tooth,  and  at  the  tip  of  a  root  is  a  small 
opening  through  which  nerves  and  blood-vessels  enter, 
The  hard  outer  part  of  a  tooth  consists  of  the  enamel,  and  the 
main  bulk  is  dentine,  a  kind  of  ivory.  The  roots  of  the 
tooth,  which  are  buried  in  holes  or  sockets  of  the  jaw-bone, 
are  covered  with  a  thin  layer  of  bony  substance,  called 
cement. 

387.  The  Tongue.  —  Examine  with  a  hand-mirror.     The 
elevations  on  the  upper   surface  are  papilke,   and    nerve- 
fibers   connect   these   with   the   brain.      Their   function   is 
that  of  taste  and  touch.     The  tongue  is  chiefly  muscular 
tissue,  the    muscle-fibers    extending    longitudinally,    trans- 
versely, and  perpendicularly.     Can  you  think  of  any  relation 
between  such  arrangements  of  the  fibers  and  the  possible 
movements  of  the  tongue  ? 

388.  Salivary    Glands.  —  The     epithelium    which     lines 
the  mouth  secretes  a  limited  amount  of  thick  fluid  known  as 
mucus,   hence  it  is  called  a  mucous  membrane.     But  most 
of  the  fluid  in  the  mouth  is  saliva  from  three  pairs  of  salivary 
glands.     On  either  side  of  the  head  a  gland  lies  beneath  and 
in  front  of  the  ear.     These  are  the  parotid  (meaning  beside  the 
ear)  glands,  and  a  disease  affecting  them  is  called  parotitis  * 
or  mumps.     A  duct  from  each  parotid  gland  opens  into  the 
mouth-cavity  on  a  little  elevation  on  the  inside  of  the  cheek 
near  the  grinding  teeth.     The  elevation  can  be  seen  by 
using  a  mirror  and  holding  the  cheek  away  from  the  jaw. 
The  two  other  pairs  of  salivary  glands  are  placed  among  the 
the  muscles  beneath  the  tongue,  and  their  ducts  open  into 

*  Notice  that  the  ending  itis  added  to  the  name  of  the  organ  (parotid) 
means  inflammation  or  disease  of  the  organ.  Likewise,  there  are  in  common 
use  such  words  as  appendicitis  (inflammation  of  the  appendix  of  the  intestines), 
gastritis  (of  the  stomach),  laryngitis  (of  the  larynx),  tonsilitis  (of  the  tonsils), 
and  many  other  diseases  designated  by  adding  itis  to  the  name  of  the  organ 
involved. 


468 


APPLIED   BIOLOGY 


I  x 


the  mouth-cavity  beneath  the  tip  of  the  tongue.  The  se- 
cretions of  the  salivary  glands  will  be  described  later  when 

we  study  their  work  in  connec- 
tion with  the  digestion  of  foods. 
389.  The  Pharynx  or  Throat- 
cavity.  —  Again  using  the  hand- 
mirror,  notice  that  at  the  back 
part  of  the  mouth-cavity  muscu- 
lar folds  extend  downward  from 
the  soft  palate  and  bound  later- 
ally the  passage  from  the  mouth- 
cavity  to  the  throat-cavity.  Be- 
tween these  folds  on  each  side  is 
a  round  body,  tonsil.  These  are 
the  organs  which  often  become 
enlarged  during  a  "  cold  "  in  the 
throat  (tonsilitis) .  They  are  not 
known  to  have  any  essential 
function,  and  surgeons  frequently 
remove  them  when  they  become 
permanently  enlarged. 

The  pharynx  opens  above  into 
the  post-nasal  cavity  behind  the 
FIG.  156.    Diagrams  showing  so^  palate;  and  below  into  the 

relations  of  respiratory  and  ali-    eSOphagUS     and    the     trachea    or 
mentary  passages  in  a  fish  (7),        •     i    •  T»    f        i       i 

amphibian  (//),  reptile  or  bird  windpipe.      Refer  back  to  your 
(///),  and  man  (IV).   Arrow  study  of  the  throat  of  the  frog. 

orZ T  rra^ntro?    A1S°'  there  °P6n  int°  the  Pha^X 

gans ;  N,  nostrils ;  K,  gill-slits ;  the   Eustachian  tubes  from  the 

D,  alimentary  canal ;  L  lungs;    earg>      The  pharynx  is  the  Central 
O,  esophagus ;  T,  trachea ;  V,  .        . 

backbone.  (After Wiedersheim.)   passage  for   communication   be- 
tween the  nose-cavities  and  the 

trachea,  and  between  the  mouth-cavity  and  the  esophagus, 
thus  providing  for  movement  of  air  from  the  nose  to  the 
lungs  and  of  food  from  the  mouth  to  the  esophagus. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       469 


390.  The  Esophagus  or  Gullet  is  a  tube  extending  from 
the  pharynx  to  the  stomach.     In  the  human  body,  and  in  all 
mammals,  it  extends  through   the 

diaphragm,  the  membrane  which 
divides  the  body-cavity  into  tho- 
racic and  abdominal  portions.  In 
the  neck  the  esophagus  is  behind 
(dorsal  to)  the  trachea. 

391.  The  Stomach  is  a  muscular 
and   greatly  expanded    portion    of 
the  alimentary  canal   or   digestive 
tube    between   the   esophagus   and     Ll 
the   intestine.     It  lies   on  the  left 
side  and  in  contact  with  the  liver. 
Imagine  a  membrane  (diaphragm) 
stretched   across   the   frog's  body- 
cavity  so  as  to  separate  the  heart 
and  lungs  from  the  stomach  and 
liver.     The  human  diaphragm  lies 

in  the  same  position ;  that  is,  the 
heart  and  lungs  lie  above  it  (ante- 
rior) and  the  stomach  and  liver  lie 
just  beneath  (posterior).  The  end 
of  the  stomach  connected  with  the 
intestine  is  provided  with  a  muscu- 
lar ring  (pylorus)  which,  by  opening 
and  closing,  is  able  to  control  the 
passage  of  food  from  the  stomach 
to  the  intestine.  It  should  be  re- 
membered that  the  size  of  a 
stomach,  like  that  of  a  rubber  bag, 
depends  upon  the  amount  of  disten- 
tion.  When  empty,  it  is  contracted  so  that  it  has  practi- 
cally no  cavity,  and  when  much  distended  by  food,  it  may 
hold  about  a  half-gallon. 


y 

FIG.  157.  Human  alimen- 
tary organs.  Gls,  salivary 
glands  ;  Ph,  pharynx  ;  Gl. 
th,  thyroid  gland  ;  Gl.  thy, 
thymus ;  Lg,  lung ;  Oe, 
esophagus  ;  Z,  diaphragm  ; 
mg,  stomach  ;  Pa,  pancreas  ; 
Lb,  liver  ;  Dd,  small  intes- 
tine ;  Vic,  valve  between 
small  and  large  intestine  ; 
Pv,  appendix  ;  Ca,  Ct,  Cd, 
colon  of  large  intestine  ;  R, 
rectum  ;  A,  anus.  (From 
Wiedersheim.) 


470  APPLIED  BIOLOGY 

392.  The   Small  Intestine.  —  The  part  of  the  intestine 
connected  with  the  stomach  is  smaller  in  diameter  than  the 
extreme  posterior  part,  and  hence  is  called  the  small  intestine. 
It  is  very  much  coiled,  as  shown  in  the  center  of  Fig.  157, 
and  is  about  twenty  feet  long. 

393.  The  Large  Intestine  is  about  five  feet  long,  which 
is  one-fourth  the  length  of  the  small  intestine ;  but  the  name 
refers  to  the  larger  diameter.     As  shown  in  Fig.  157,  the 
large  intestine  extends  upward,  from  the  point  of  union  with 
the    small    intestine,    then    transversely,    then    downward. 
This  largest  portion  is  often  called  the  colon.     The  terminal 
or  posterior  portion  of  the  large  intestine,  commonly  called 
the  rectum,  is  smaller  in  diameter  than  the  colon  part ;   and 
its  external  opening  is  the  anus.     Between  the  colon  and  the 
rectum  there  is  an  S-shaped   loop   (sigmoid   flexure);  and 
near  the  junction  of  the  large  and  small  intestines  is  the 
vermiform  appendix  (or  simply  appendix)  a  tube  from  two 
to  four  inches  long  and    one-fourth  an  inch  in  diameter. 
Its  inflammation,  due  to  bacteria,  is  appendicitis,  for  which 
the  usual  cure  is  surgical  removal  of  the  organ.     It  has  no 
function  in  man ;   but  in  the  rabbit  and  many  lower  herbiv- 
orous animals  it  is  large  and  important  in  digestion. 

394.  Liver    and    Pancreas.  —  These    two    organs    should 
be  named  in  the  list  of  digestive  organs,  for  they  secrete 
fluids  which  are  poured  into  the  intestine  by  ducts.      The 
position  of  the  liver  has  been  described  in  connection  with  the 
stomach.     The  pancreas,  which  in  the  case  of  some  animals 
used    as    human    food    is    called    "  stomach-sweetbread,"* 
lies  near  the  junction  of  the  stomach  and  the  small  intestine. 
Its  main  'duct  (pancreatic  duct)  joins  the  bile-duct  from  the 
liver,  and  the  fluids  secreted  by  the  two  organs  are  poured 
into  the  intestine  a  short  distance  from  the  stomach. 


*  The  neck-  or  throat-sweetbread  sold  in  meat-markets  is  from  the 
thymus,  an  organ  found  only  in  young  animals,  such  as  calves  and  lambs, 
and  lying  in  the  anterior  part  of  the  chest-cavity  close  to  the  neck. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       471 

395.  Glands.  —  The  liver,  pancreas,  salivary  glands,  and 
gastric  glands  have  been  mentioned  as  producers  of  special 
secretions  useful  in  digestion.     Microscopic  study  shows  that 
a  gland  is  composed  of  a  layer  of  epithelial  cells  which  rest 
on  a  bed  of    connective  tissue.      In  this  latter  tissua  are 
blood-vessels  which  supply  food  and  oxygen  to  the  cells 
of  the  glands;  and  from  the  materials  thus  obtained  the 
cells   manufacture   their   secretions,    which   they   discharge 
at  the  free  end  of  the  cells  (i.e.,  opposite  the  end  where  they 
may  absorb  from  the  blood).     See  Figs.  158,  159. 

(D)  Sections  of  frog's  stomach  show  structure  of  simple  gland. 
Pancreas  sections  show  numerous  tubes  cut  across  at  various  angles. 

THE  WORK  OF  THE  DIGESTIVE  ORGANS 

We  have  briefly  considered  the  structure  of  the  digestive 
organs,  and  it  is  now  our  problem  to  study  the  work  of  these 
organs,  especially  with  reference  to  the  preparation  for  ab- 
sorption of  the  different  kinds  of  food;  i.e.,  the  nutrients. 
As  in  the  frog,  this  digestion  of  foods  is  caused  by  secretions  ; 
and  in  the  human  body  these  are  saliva  from  the  salivary 
glands,  gastric  juice  from  the  glands  in  the  wall  of  the  stomach, 
intestinal  juice  from  the  glands  in  the  wall  of  the  intestine, 
pancreatic  juice  from  the  pancreas,  and  bile  from  the  liver. 

396.  Mechanical  Processes  in  Digestion.  —  Various  move- 
ments, due  to  the  action  of  muscles  in  the  digestive  organs, 
play  two  important  parts :  (1)  in  taking  food  and  in  mov- 
ing it  along  through  the  digestive  tube,  and  (2)  in  separat- 
ing more  or  less  solid  food  into  small  particles  upon  which  the 
digestive  fluids  can  act  rapidly.     The  muscular  action  of 
the  lips  and  jaws  in  taking  and  chewing  (masticating)  food 
is  so  easily  observed  that  no  description  is  necessary.     The 
mastication  process  is  important  in  that  it  results  in  mixing 
saliva  with  the  food  (see  §  455,  on  hygiene  of  eating). 

After    mastication    comes    the    swallowing    movements, 


472  APPLIED   BIOLOGY 

which  are  produced  by  the  muscles  of  the  pharynx  and  esoph- 
agus. These  movements  are  in  the  beginning  voluntary 
(i.e.,  under  control  of  the  will) ;  but  it  is  well  known  that 
when  food  has  started  down  the  esophagus,  the  muscular 
walls  of  that  organ  contract  (narrow  the  diameter)  involun- 
tarily, and  force  the  food  into  the  stomach.  The  fact  that 
food  is  forced  down  and  does  not  run  because  of  gravity  ex- 
plains how  an  acrobat  can  perform  the  feat  of  drinking  water 
while  standing  on  his  head.  Horses  and  cows  are  examples 
of  animals  which  regularly  drink  and  eat -with  the  mouth- 
cavity  much  lower  than  the  stomach. 

(D)  Take  a  piece  of  rubber  tubing  about  two  feet  long,  suspend 
vertically,  plug  the  lower  end  with  a  cork,  and  fill  with  water.  Now 
begin  at  the  plugged  end  and  grasp  the  tubing  between  a  thumb  and 
finger  and  then  move  your  hand  upward  so  as  to  contract  the  tube 
in  succession.  It  is  evident  why  the  water  appears  to  run  up-hill  in 
the  tube.  Similarly,  the  muscles  which  are  in  the  walls  of  the 
esophagus  contracting  in  succession  from  the  pharynx  toward  the 
stomach  force  the  food  along  the  esophagus. 

The  muscle-fibers  in  the  walls  of  the  stomach  are  arranged 
transversely,  longitudinally,  and  obliquely.  The  part  of 
the  stomach  next  to  the  esophagus  contracts  and  steadily 
presses  upon  the  contained  food;  but  the  posterior  half 
undergoes  a  series  of  wave-like  constrictions.  The  result 
is  that  the  food  is  broken  up  into  smaller  masses  and  well 
mixed  with  the  digestive  fluids.  These  movements  have 
been  studied  by  feeding  dogs  and  cats  with  bismuth  sub- 
nitrate,  a  drug  which  physicians  often  prescribe  to  allay 
gastric  irritation.  This  drug  mixes  with  the  food  in  the 
stomach  and  renders  the  whole  mass  so  opaque  to  the  X-rays 
that  the  shape  of  the  stomach  is  easily  seen  with  the  aid  of 
a  fluoroscope,  and  photographs  can  be  made  similar  to  those 
often  taken  of  the  skeleton  within  the  body. 

From  time  to  time  the  pylorus  opens,  allowing  liquid 
foods  (often  called  chyme)  to  pass  into  the  intestine.  As 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       473 

a  rule,  the  large  masses  of  solid  food  are  held  back  in  the 
stomach. 

The  movements  of  the  intestine  consist  of  constrictions. 
These  sometimes  appear  in  series  at  certain  points,  and  then 
next  at  the  points  which  are  halfway  between  the  first  con- 
strictions. This  causes  a  thorough  mixing  of  the  foods  with 
the  digestive  fluids.  Another  form  of  constriction  which 
occurs  at  intervals  moves  along  the  intestine  and  forces  the 
contents  towards  the  large  intestine.  The  above  experiment 
with  rubber  tubing  illustrates  this  type  of  constriction.  Such 
movements  are  known  as  peristaltic  contractions. 

397.  Why  Foods  must  be  Digested.  —  We  have  seen  in 
our  study  of  the  frog  that  foods  must  be  prepared  so  that 
they  may  be  absorbed  through  the  lining  membranes  (epi- 
thelium) of  the  digestive  organs  into  the  blood.  Jn  other 
words,  foods  must  be  capable  of  osmosing  through  mem- 
branes ;  and  most  foods,  as  taken  into  the  mouth,  are  not 
ready  for  this  process.  Sugar  and  common  salt  dissolved 
in  water  are  practically  the  only  cases  of  common  food  which 
can  osmose  or  be  absorbed  without  digestion.  Even  milk, 
our  most  common  liquid  food,  contains  droplets  of  fat  too 
large  for  absorption  and  which  must  be  dissolved  in  prepara- 
tion for  absorption. 

(D)  Microscopic  Examination  of  Milk. —  Mount  a  drop  of  milk  on 
a  glass  object-slide.  Examine  with  (1)  low-power,  (2)  high-power 
objective.  Note  the  droplets  of  fat.  These  are  lighter  than  the 
water  of  the  milk ;  and,  like  oils  in  general,  rise  to  the  surface  when 
milk  stands,  forming  the  concentrated  layer  of  fat  which  we  call 
cream.  When  the  cream  is  shaken  or  agitated,  as  in  a  churn,  these 
fat  droplets  fuse  together  into  larger  masses  of  fat,  which  we  call 
butter. 

Since  digestion  practically  means  the  preparation  of  foods 
for  absorption,  it  is  convenient  to  study  the  action  of  the 
various  digestive  secretions  (saliva,  gastric  juice,  bile,  and 
pancreatic  fluid)  by  adding  them  to  foods  in  test-tubes ;  and 


474  APPLIED  BIOLOGY 

after  allowing  time  for  a  change,  try  for  osmosis  of  the  food 
through  membranes,  such  as  fish-bladder  or  parchment. 
There  are  good  reasons  for  thinking  that  any  food  which  will 
osmose  through  a  dead  membrane  will  also  be  absorbed  under 
conditions  which  exist  in  the  linings  of  the  stomach  and 
intestine.  Hence  any  digestive  changes  which  will  prepare 
foods  in  test-tubes  for  osmosis  will  serve  to  illustrate  the 
changes  which  take  place  inside  the  living  alimentary  canal. 
But  not  all  the  digestive  processes  can  be  illustrated  by  test- 
tube  experiments,  for  the  living  digestive  organs  cause  some 
changes  in  foods  which  cannot  be  imitated  in  the  lifeless 
conditions  in  test-tubes.  However,  it  is  possible  to  perform 
a  number  of  experiments  which  will  throw  much  light  on  the 
various  secretions  and  parts  of  the  alimentary  canal  with 
regard  to  their  part  in  the  digestion  of  the  various  kinds  of 
foods.  We  shall  consider  the  digestive  processes  in  the 
order  in  which  foods  come  in  contact  with  the  secretions  in 
passing  from  the  mouth-cavity  through  the  esophagus  into 
the  stomach,  and  thence  into  the  intestine ;  this  means  con- 
tact, in  succession,  with  the  salivary,  gastric,  intestinal, 
pancreatic,  and  hepatic  (liver)  secretions. 

398.  Digestion  by  Saliva.  —  The  fluid  secreted  by  the 
salivary  glands  consists  chiefly  of  water  and  an  enzyme 
called  ptyalin,  which  is  very  similar  to  the  diastase  of  plants. 
The  dry  ptyalin  may  be  purchased  from  chemists,  or  saliva 
may  be  collected  in  a  test-tube  (when  chewing  a  piece  of 
rubber  or  gum  it  is  secreted  rapidly).  The  work  of  saliva  is 
the  digestion  of  starch  to  a  kind  of  sugar,  which  is  absorbable. 
This  is  illustrated  by  the  following  experiments:  — 

(D)  Make  some  thin  starch  paste,  by  heating  starch  in  water. 
Notice  that  the  resulting  fluid  is  not  clear,  but  opalescent.  Place  a 
small  quantity  of  the  paste  in  a  test-tube  and  add  a  few  drops  of 
iodine-solution.  Note  the  color. 

(D)  Place  some  of  the  paste  in  a  small  bag  made  from  a  piece  of 
gold-beaters'  membrane,  or  fish-bladder,  and  suspend  the  bag  so 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       475 

that  it  dips  into  water  in  a  small  tumbler  or  beaker.  Or  put  some 
of  the  paste  in  an  osmose-apparatus.*  After  allowing  an  hour  or 
two  for  osmosis  to  take  place,  transfer  some  water  from  the  tumbler 
into  the  test-tube,  and  add  some  iodine  solution.  Compare  with 
test  of  starch  as  directed  above.  Is  there  evidence  that  starch 
osmoses  through  a  membrane? 

(D)  Along  with  the  above,  test  grape-sugar  or  corn-syrup  by 
placing  in  water  inside  a  bag  or  in  the  tube  of  the  osmose-apparatus. 
Wait  the  same  time  as  in  the  case  of  the  starch,  and  test  water  in  the 
tumbler  with  Fehling's  reagent  (§  102).  Does  the  sugar  osmose  ? 

(L)  Mix  some  starch  scraped  from  a  potato  in  a  drop  of  cold 
water  on  a  glass  slide,  put  on  cover-glass,  and  examine  with  low 
power  of  microscope.  Note  the  appearance  of  the  starch-grains. 
Now  remove  the  slide  from  the  microscope  and  heat  slowly  over  an 
alcohol-  or  a  gas-burner  until  the  water  begins  to  steam.  Examine 
again  with  the  microscope,  and  note  the  broken  starch-grains.  These 
broken  grains  mixed  with  the  water  form  starch  paste,  and  the  opales- 
cent appearance  of  the  thin  paste  is  due  to  the  numerous  particles 
of  starch  floating  in  the  water.  Examine  starch  scraped  from  a  baked 
or  boiled  potato. 

(D)  Place  some  thin  starch  paste  in  each  of  two  test-tubes. 
Add  to  one  tube  (No.  1)  a  few  drops  of  clear  saliva  (filtered  through 
coarse  filter  paper).  Tube  No.  2  has  only  paste.  After  twenty 
to  forty  minutes  note  the  appearance  of  the  paste  in  the  tubes, 
especially  with  regard  to  opalescence.  Pour  a  small  quantity  from 
tube  with  the  saliva  (No.  1)  into  a  clean  test-tube,  and  add  a  few 
drops  of  iodine  solution.  Is  starch  present  ? 

(D)  Into  another  tube  pour  some  starch  paste  from  tube  No.  1, 
and  into  still  another  pour  paste  that  has  been  acted  upon  by  saliva 
in  tube  No  2.  To  each  of  the  tubes  apply  Fehling's  test.  Re- 
sults? Conclusions?  Do  these  experiments  suggest  why  a  dry 
starchy  cracker  becomes  sweet  to  the  taste  after  being  held  in  the 
mouth  for  some  time? 

(D)  If  time  permits,  prepare  three  tubes  with  thin  starch  paste. 
To  No.  1  add  boiled  saliva ;  to  No.  2  add  normal  saliva,  but  add  a 
few  drops  of  acid  (vinegar  will  do)  to  make  the  paste  slightly  acid 
(test  with  blue  litmus-paper) ;  and  to  No.  3  add  saliva,  but  keep  the 
tube  standing  in  a  tumbler  filled  with  finely  cracked  ice.  After 
twenty  to  forty  minutes  test  for  starch  and  sugar  as  in  above. 
What  do  these  experiments  show  regarding  (1)  effect  of  boiling 


*  Described  in  "  Teachers'  Manual.' 


476  APPLIED  BIOLOGY 

saliva,  (2)  effect  of  acid,  (3)  effect  of  low  temperature?  Boiling 
and  low  temperature  have  the  same  effect  on  all  the  secretions  of  the 
stomach,  intestine,  and  pancreas. 

(D)  In  order  to  show  that  saliva  changes  starch  into  sugar  capable 
of  being  absorbed  (i.e.,  osmose  through  a  membrane),  take  some 
starch  paste  which  has  been  acted  upon  (digested)  by  saliva  for  two 
or  three  hours,  place  in  a  membrane  bag  or  osmose-apparatus,  allow 
half-hour  for  osmose,  then  test  water  in  tumbler  for  sugar. 

The  above  experiments  simply  prove  that  saliva  digests 
starch  to  a  sugar;  and  that  while  starch  does  not  osmose, 
the  sugar  formed  from  starch  does.  Saliva,  then,  prepares 
starch  for  absorption  into  the  blood  which  flows  in  blood- 
capillaries  beneath  the  lining  membrane  (epithelium)  of  the 
alimentary  canal,  especially  abundant  in  the  stomach  and 
intestines.  The  sugar  derived  from  starch  osmoses  through 
the  epithelium  into  the  blood,  just  as  it  is  demonstrated  by 
the  above  experiments  that  it  will  osmose  through  the  dead 
membrane  of  the  osmose-apparatus. 

It  is  evident  that  the  above  experiments  do  not  prove 
anything  regarding  the  action  of  saliva  on  other  nutrients. 
If  time  permitted,  we  might  try  similar  experiments  with 
proteins  and  fats,  using  these  nutrients  in  place  of  starch, 
and,  of  course,  applying  the  appropriate  tests  (§  377).  Such 
experiments  have  been  performed  many  times  by  physi- 
ologists, and  their  conclusion  is  that  saliva  has  no  digestive 
power  for  foods  other  than  starch. 

No  simple  experiment  will  show  that  there  is  in  saliva  a 
substance  (ptyalin)  which  causes  the  digestion  of  starch,  but 
chemists  have  proved  it  to  be  present,  and  the  cause  of  the 
action  of  saliva.  Moreover,  it  has  been  shown  to  be  an 
enzyme,  because  it  acts  by  its  presence,  without  entering 
into  the  sugar  formed,  and  a  very  small  amount  of  it  can 
digest  a  large  quantity  of  starch. 

The  amount  of  starch  food  which  will  be  digested  in  the 
mouth-cavity  depends  upon  the  length  of  time  it  is  held 
there.  Hence,  prolonged  mastication  is  advisable.  It  has 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       477 

been  demonstrated  within  recent  years  that  saliva  continues 
its  action  on  starch  for  some  minutes  after  the  food  reaches 
the  stomach,  or  until  the  acid  gastric  juice  stops  the  action. 

399.  Digestion  by  Gastric  Juice.  —  Physiologists  have 
found  that  the  human  stomach  secretes  from  five  to  ten 
quarts  of  gastric  juice  in  twenty-four  hours.  This  secretion 
consists  chiefly  of  water  containing  a  small  amount  of  hydro- 
chloric acid  and  three  enzymes  —  pepsin,  rennin,  and  lipase. 
The  last  named  seems  to  be  of  most  importance  after  food 
reaches  the  intestines.  The  action  of  pepsin  and  rennin  are 
most  easily  demonstrated.  Pepsin  may  be  extracted  by 
soaking  stomach-membranes  in  glycerine,  but  the  commercial 
extract  sold  at  drug-stores  is  most  convenient  for  experi- 
ments. Rennin  is  especially  abundant  in  the  stomachs  of 
calves,  and  is  sold  in  grocery  and  drug  stores  under  the 
names  of  "  liquid  rennet  "  and  "  junket  tablets." 

Numerous  experiments  by  physiologists  have  proved  that 
gastric  juice  digests  proteins,  curdles  (coagulates)  milk,  dis- 
solves some  minerals  in  foods,  digests  a  small  amount  of  fat, 
but  it  has  no  effect  upon  starch.  In  this  course,  we  have 
time  for  only  a  few  experiments  which  illustrate  some  of 
these  points. 

(D)  The  action  of  pepsin  on  proteins.  —  (1)  Make  some  albumen- 
solution  by  mixing  white-of-egg  in  cold  water,  fill  a  test-tube  half 
full,  add  some  dry  pepsin  or  glycerine  extract  of  pepsin,  add  enough 
hydrochloric  acid  to  make  the  mixture  slightly  acid  to  litmus  paper, 
place  the  tube  in  a  warm  place  near  a  stove  or  radiator,  or  in  a  "fire- 
less  cooker,"  or  bucket  of  water  heated  to  37°  C.  (98  F.)  and  pro- 
tected from  cooling  rapidly.  After  from  three  to  ten  hours,  pour  the 
contents  of  the  test-tube  into  a  membrane  bag  or  osmose-apparatus 
and  suspend  in  pure  water.  (Start  the  next  experiment  at  this  time.) 
Allow  an  hour  for  osmosis.  Test  the  water  for  proteins  (§  377), 
using  Millon's  reagent.  Have  proteins  gone  through  the  membrane 
into  the  water? 

(2)  Pour  some  fresh  undigested  albumen-solution  into  a  membrane 
bag,  and  after  an  hour  test  for  osmosis  as  in  above  experiment. 
Does  this  albumen  osmose  ?  What  conclusion  is  to  be  drawn  con- 


478 


APPLIED  BIOLOGY 


cerning  the  effect  of  pepsin  and  acid  on  the  particular  protein  used 
in  these  experiments  ? 

If  time  permits,  other  proteins  may  be  tested  in  the  same  way. 
Try  a  hard-boiled  egg  grated  or  minced  into  fine  particles,  shreds  of 
lean  meat,  or  cheese.  Also,  the  acid  may  be  omitted,  in  order  to 
show  that  pepsin  alone  will  not  digest  proteins. 

(D)  The  action  of  the  rennin  may  be  illustrated  by  adding  some 
commercial  liquid  rennet  or  junket  tablets  to  milk.  Place  some  of 
the  curds  on  filter-paper,  wash  with  water,  place  in  a  test-tube  with 
some  water,  add  strong  nitric  acid,  heat.  What  nutrient  is  abundant 
in  the  curds  ?  Pepsin  and  acid  will  digest  them. 

Summarizing,  gastric  juice  can  digest  proteins,  coagulate 
the  proteins  of  milk,  and  then  digest  them,  and  digest  a 

small  amount  of  fat ;  but 
it  does  not  digest  starch. 
Fat  meat  breaks  up  ex- 
tensively when  kept  for 
some  hours  in  gastric 
juice,  but  only  the  pro- 
tein walls  of  the  cells  have 
been  digested,  allowing 
the  contained  oil  to  es- 
cape undigested. 

Fat-cells.  —  (D)  Place  a 
small  piece  of  fat  meat  or 
suet  on  an  object-slide  in  a 
drop  of  glycerine,  and  with 
a  pair  of  needles  tease  the 
meat  into  as  small  particles 
as  possible.  Put  on  cover- 
glass,  and  examine  with  low 
and  high  power  of  a  micro- 
scope. Note  the  spherical 
cells  filled  with  fat,  which 


FIG.  158.  Part  of  a  cross  section  of  intes- 
tine of  a  mammal,  x,  marks  center  of 
cavity  of  intestine  ;  g,  intestinal  gland 
between  villi ;  /,  lymph  gland  ;  v,  villus  ; 
ra,  muscle  layers,  the  outer  being  longi- 
tudinal and  the  inner  circular  in  arrange- 
ment of  fibers  (cells) ;  6,  blood-vessels  ; 
ly,  a  lacteal  or  lymph-vessel  in  center 
of  a  villus  (v) ;  a,  peritoneum.  Compare 
with  Fig.  19  of  frog.  (From  Wieder- 
sheim.~) 


may  be  in  the  form  of  needle- 
shaped  crystals  at  the  ordinary  temperatures.  If  a  small  piece  of  fat 
meat  be  soaked  for  a  time  in  ether  to  dissolve  the  fat,  and  then  placed 
iniodine-eosm  or  other  stains,  the  nuclei  of  the  fat-cells  will  be  stained. 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       479 


C-J 


400.  Digestion  in  the  Intestine.  —  While  the  gastric  juice 
has  power  to  digest  protein  foods,  the  fact  is  that  most  of 
them  do  not  remain  in  the 

stomach  long  enough  for 
complete  digestion  ready 
for  absorption.  At  inter- 
vals the  muscular  ring  of 
the  pylorus  relaxes  and 
allows  partially  digested 
foods  to  escape  into  the 
intestine,  there  to  undergo 
final  digestion  and  absorp- 
tion into  the  blood.  This 
food  which  escapes  into 
the  intestine  from  the 
stomach  is  water  contain- 
ing (1)  much  starch  not 
digested  by  the  saliva,  (2) 
sugar  which  was  eaten  as 
such  and  much  of  the 
sugar  formed  by  the  di- 
gestion of  starch  by  the  &LM 
saliva,  (3)  dissolved  pro- 
teins digested  in  the 
stomach,  (4)  many  small 
particles  of  undigested 
proteins,  (5)  fat  in  liquid  FIG.  159. 
(oil)  condition.  The  work 
of  the  intestine  is  to  com- 
plete the  digestion  of  the 
proteins,  fats,  and  starch; 
and  to  absorb  the  products  of  digestion  into  the  blood. 

401.  Secretions  in  the  Intestine.  —  These  are  formed  by 
the  pancreas,  the  liver,  and  the  glands  of  the  intestinal  walls. 
The  pancreatic  secretion  contains  enzymes  able  to  digest 


7..»tr* 


A,  two  villi  from  intestine. 
Blood-vessels  represented  by  black  lines. 
g .1,  intestinal  gland  ;  I,  lacteal  or  lymph- 
vessel  ;  B,  three  cells  from  covering  of 
villi ;  C,  two  cells  from  lining  of  glands. 
(After  Hardy.) 


480  APPLIED  BIOLOGY 

any  kind  of  food,  and  consequently  any  food  not  made  ready 
for  absorption  while  in  the  stomach  is  digested  by  the  en- 
zymes poured  into  the  intestine.  The  final  result  is  that  the 
starch,  proteins,  and  fat  which  enter  the  intestine  undigested 
are  prepared  for  absorption.  For  this  work  the  intestine 
is  specially  adapted,  because  of  the  numerous  delicate  pro- 
cesses (called  villi,  singular  is  villus)  which  extend  from  the 
lining  membrane  (see  Fig.  159).  These  are  richly  supplied 
with  blood-  and  lymph-vessels,  into  which  digested  food  is 
absorbed ,  rapidly. 

402.  Details  of  Digestion  in  Intestine  (Optional). — The 
bile  from  the  liver,  the  secretion  from  the  glands  in  the  walls 
of  the  intestine,  and  the  pancreatic  secretion  are  alkaline, 
and  quickly  change  the  acid  food  which  arrives  from  the 
stomach.  Of  the  pancreatic  secretion,  the  enzyme  called 
amylopsin  acts  like  the  ptyalin  of  the  saliva  and  digests  starch 
to  sugar ;  the  enzyme  called  trypsin  acts  on  proteins  much 
the  same  as  pepsin  does  in  the  stomach ;  and  the  enzyme 
lipase  (or  steapsin)  changes  fats  to  fatty  acid  and  glycerine. 
The  fatty  acid  easily  combines  with  the  alkaline  substances 
in  bile  and  intestinal  juice  and  forms  a  kind  of  soap  which 
is  absorbable.  Since  the  pancreatic  secretion  is  able  to 
digest  rapidly  the  three  kinds  of  food,  it  is  by  far  the  most 
important  digestive  secretion. 

Bile,  from  the  liver,  does  not  itself  digest  any  food,  but  it 
has  been  shown  that  fats  are  digested  and  absorbed  better 
when  bile  is  present. 

The  intestinal  juice,  from  the  glands  of  the  intestine,  has 
a  number  of  enzymes  of  which  the  most  important  change 
cane-sugar,  milk-sugar,  and  malt-sugar  into  glucose  or  similar 
sugars. 

(D)  It  is  possible  to  demonstrate  the  effect  of  pancreatic  secre- 
tion upon  proteins  and  starch  by  performing  experiments  similar 
to  the  previous  ones  (§§  398  and  399)  with  saliva  and  gastric  juice, 
but  substituting  in  this  case  pancreatic  secretion.  This  may  be 


HUMAN   STRUCTURE  AND  LIFE-ACTIVITIES       481 

obtained  by  crushing  some  fresh  pancreas  and  soaking  in  glycerine 
as  suggested  in  §  399  for  extracting  pepsin ;  or  commercial  prepara- 
tions of  pancreas  extract  may  be  obtained  at  drug-stores.  Add 
either  the  glycerine  extract  or  the  commercial  pancreatic  extract  to 
water  in  test-tubes  and  also  enough  sodium  carbonate  to  make  the 
mixture  alkaline  (as  shown  by  red  litmus  paper).  Starch  or  protein 
may  be  digested  in  this  solution  for  one  or  more  hours,  with  tempera- 
ture between  35  and  40°  C.,  and  then  tested  for  osmosis  as  suggested 
in  §  398.  Extract  of  pancreas  does  not  furnish  very  active  lipase, 
and  hence  it  is  difficult  to  demonstrate  the  action  on  fats. 

403.  Summary  of  Digestion  and  Absorption  of  Foods.  — 
Grape-sugar  dissolved  in  water  may  be  very  slightly  absorbed 
in  mouth-cavity  and  esophagus,  some  in  stomach,  but 
chiefly  in  intestine.  Cane  and  other  sugars  are  changed 
before  absorption  in  the  intestine. 

Starch  —  some  digestion  to  sugar  (maltose)  in  mouth- 
cavity  and  in  stomach  by  saliva',  but  chiefly  digested  to 
maltose  in  intestine  by  amylopsin  of  the  pancreatic  secre- 
tion. Resulting  sugar  may  be  absorbed  as  stated  for  sugar 
above. 

Proteins  —  digested  to  absorbable  form  in  stomach  by 
pepsin  and  in  intestine  by  pancreatic  secretion.  Absorbed 
to  slight  extent  from  stomach,  but  chiefly  from  intestine. 
Milk  proteins  are  coagulated  in  stomach  by  rennin  as  a  special 
preparation  for  digestion  by  action  of  pepsin,  or  later  by 
pancreatic  secretion. 

Fats.  —  Chiefly  digested  in  the  intestine  by  lipase  of  pan- 
creatic secretion,  aided  by  bile  and  intestinal  juice.  Slightly 
digested  by  lipase  in  the  stomach.  The  protein  part  of 
tissues  containing  fat  (e.g.,  bacon  or  suet)  is  digested  away 
in  stomach  by  pepsin,  thus  freeing  the  liquid  fat. 

Mineral  foods.  —  Those  that,  like  common  salt,  are  soluble 
in  water  require  no  change.  Others  present  in  various  animal 
and  vegetable  foods  which  we  commonly  use  are  easily  dis- 
solved in  the  acid  gastric  juice.  All  the  mineral  foods  are 
ready  for  absorption  when  dissolved  in  water. 
2i 


482  APPLIED  BIOLOGY 

404.  Effect  of  Cooking  Foods.  —  Cooking  not  onl/renders 
foods  more  palatable,  but  also  more  digestible.     It  is  easily 
observed  that  meats  and  vegetables  are  softened  by  cooking 
processes,  and  this  aids  the  penetration  of  digestive  juices. 
See  effect  of  boiling  upon  starch  (§  398) .     Roasting,  broiling, 
boiling,    and   steaming   best   prepare   foods   for   digestion; 
while  frying  in  lard,  butter,  or  oil  causes  the  oily  materials  to 
penetrate  the  food  and  thus  render  the  penetration  of  diges- 
tive juices  more  difficult.     In  general,  a  similar  result  comes 
from  any  heating  of  starch  and  fat  together;  and  hence 
pastries,  such  as  pie-crust,  are  more  indigestible  than  would 
be  the  same  amount  of  the  ingredients  taken  into  the  stomach 
without    heating    together.     Various    methods    of    making 
starchy  foods  porous,  such  as  the  action  of  baking-powder 
and  yeast,  favor  rapid  penetration  by  the  digestive  juices. 

405.  Transportation  of  Digested  Food  to  All  Cells.  —  It 
has  been  slated  in  a  general  way  that  foods  are  required  by 
all  the  living  cells  of  the  human  body.     The  study  of  diges- 
tion and  absorption  has  shown  how  foods  get  into  the  blood, 
and  now  we  want  to  know  how  foods  get  from  the  blood  into 
the  cells.      This  requires  (1)  that  blood  should  be  moved 
from  the  capillaries  in  the  walls  of  the  digestive  organs  to 
the  capillaries  in  all  the  other  organs  of  the  body,  and  (2)  then 
absorption  from  the  blood  by  cells  which  need  food.     Before 
one  can  understand  how  food  is  transported  to  all  parts  of 
the  body,  it  is  necessary  to  make  some  study  of  the  general 
plan  of  structure  of  the  organs  concerned  with  the  move- 
ment of  blood  and  lymph,  and  also  of  the  nature  of  these 
two  liquids.      Hence  our  next  lesson  deals  with  these  topics. 

BLOOD  AND  LYMPH 

406.  Structure  of  Blood.  —  If  examined  with  the  micro- 
scope, blood  is  found  to  be  a  liquid,  called  plasma,  in  which 
float  numerous  small  bodies,  blood-corpuscles  or  blood-cells. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       483 


About  90  per  cent  of  the  plasma  consists  of  water,  which  has 
been  absorbed  from  the  digestive  organs,  and  more  than  9  per 
cent  consists  of  absorbed  foods.  There  is  also  a  small  amount 
of  excretions  which  have  been  absorbed  from  cells  and  are 
on  their  way  to  being  eliminated  by  the  excretory  organs. 

There  are  two 
kinds  of  corpus- 
cles or  cells  visible 
with  the  micro- 
scope; namely, 
the  red  and  the 
white.  The  red 
ones  are  bi-con- 
cave  discs,  about 
-g-^Vff  of  an  inch  in 
diameter.  They 
are  often  seen  in 
rolls  like  piles  of 
coins.  The  num- 
ber of  them  is  as- 
tounding, for 
there  are  about 
five  millions  in  a 
cubic  millimeter 
of  blood.  (A  mil- 
limeter is  approx- 
imately ^V  of  an 
inch  long ;  com- 
pute how  many  red  corpuscles  in  a  cubic  inch  of  blood.) 
In  the  diseased  condition  known  as  anaemia,  which  is  char- 
acterized by  the  whiteness  of  the  skin,  the  number  of  red 
corpuscles  is  greatly  reduced. 

The  red  color  of  these  blood-cells  is  due  to  a  substance 
called  hcemoglobin,  which  is  most  important  in  the  blood's 
work  of  carrying  oxygen  to  the  cells  of  the  body. 


1^0.  Human  blood-cells.  A,  rolls  of  red  cells 
with  two  white  cells  a.  B,  C,  D,  E,  various  views  of 
red  celis.  Ft  white  ceii  magnified  same  as  D.  H,  I, 
red  ce^s  covered  with  little  knobs  (abnormal,  due 
to  changes  after  blood  is  placed  on  object-slide). 

(From  *uxley,} 


484  APPLIED  BIOLOGY 

The  red  corpuscles  of  man  and  other  mammals  have  no 
nuclei,  except  in  embryonic  stages ;  but  all  lower  vertebrates 
—  birds,  reptiles,  amphibia,  and  fishes  —  have  nuclei  in  all 
red  corpuscles,  even  in  the  adult  animals.  Scientists  have 
not  yet  found  any  reason  for  this  difference. 

It  is  also  interesting  to  know  that  red  corpuscles  are  con- 
stantly being  formed  in  the  red  marrow  of  bones.  Obtain  a 
long  bone  from  a  meat-market,  break  it  open,  and  examine 
the  marrow. 

The  white  corpuscles  are  irregular  in  shape,  because  they 
move  spontaneously  like  the  Amceba  (§  270).  In  human 
blood  they  occur  in  the  proportion  of  one  white  to  700  of  the 
red  cells.  They  are  exceedingly  abundant  in  blood  taken 
from  any  inflamed  place,  such  as  a  boil,  pimple,  or  wound. 
Like  the  Amceba,  which  they  resemble,  although  there  is  no 
connection  as  to  origin,  the  white  cells  can  engulf  ("  eat  ") 
small  particles,  such  as  bacteria,  by  the  flowing  of  the  pro- 
toplasm around  the  object.  It  is  believed  that  their  work  or 
function  is  that  of  destroying  bacteria  and  particles  of  dead 
cells.  This  explains  why  they  are  abundant  in  such  places 
as  boils  and  abscesses.  The  material  known  as  pus,  which 
exudes  from  such  centers  of  inflammation,  contains  numerous 
white  corpuscles  and  many  particles  from  the  dead  cells  of 
the  inflamed  tissue. 

(D)  Examine  a  drop  of  human  blood  spread  on  an  object-slide  and 
protected  with  a  cover-glass.  A  drop  may  be  obtained  by  wrapping 
a  piece  of  string  tightly  around  a  finger  near  its  end  and  then  prick- 
ing the  skin  with  a  needle  which  is  first  made  sterile  by  passing 
several  times  through  a  flame.  One  small  drop  of  blood  should  be 
placed  in  a  drop  of  normal  salt  solution  (common  salt  7  grams  in  a 
liter  of  water,  which  gives  0.7  per  cent).  The  red  corpuscles  appear 
faint  yellow  in  color  when  not  in  masses.  The  rarer  white  cells  are 
difficult  to  find,  but  may  be  located  by  turning  the  micrometer  screw 
about  half  a  revolution  so  as  to  throw  the  red  corpuscles  slightly  out 
of  clear  focus,  and  then  a  few  white  cells  may  be  seen  as  glistening 
points  of  light  in  the  field.  They  are  somewhat  larger  than  the  red  cells. 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       485 

Blood  from  a  frog,  fish,  or  bird,  or  permanent  preparations  of 
these,  may  also  be  examined.  Nuclei  will  be  seen  in  the  red  cells 
which  are  much  larger  than  those  in  human  blood. 

Blood  from  an  earthworm,  crayfish,  insect,  or  other  invertebrate 
animal  will  show  only  white  cells. 

407.  Coagulation    of    Blood.  —  Every    one    knows    how 
blood  will  flow  from  a  cut  for  a  time  and  then  stop  because 
some  of  it  has  thickened  into  a  jelly-like  mass,  which  closes 
over  the  cut  and  forms  a  scab.    This  process  of  thickening  is 
known  in  physiology  as  coagulation,  or  clotting.     Examina- 
tion of  blood  allowed  to  clot  in  a  drop  of  normal  salt  solution 
will  show  that  coagulation  is  due  to  the  formation  of  delicate 
fibers  which  bind  the   corpuscles  into  a  semi-solid  mass. 
These  fibers  are  composed  of  a  substance  called  fibrin,  which 
is  simply  the  solid  form  of  a  protein  called  fibrinogen.     This 
is  dissolved  in  the  plasma  of  the  blood  and  becomes  hardened 
into  threads  when  blood  escapes  from  a  blood-vessel.     If 
fresh  blood  of  any  animal  be  stirred  with  a  feather  or  camel 's- 
hair  brush,  the  fibrin  will  form  on  the  feather  and  the  de- 
fibrinated  blood  will  not  coagulate,  thus  proving  that  the 
formation  of  fibrin  causes  coagulation. 

The  use  of  coagulation  is  obviously  to  prevent  excessive 
bleeding.  If  blood  did  not  coagulate  quickly,  a  slight  in- 
jury to  even  a  small  artery  might  cause  the  death  of  animals 
or  of  men  before  surgical  aid  could  be  secured. 

408.  Lymph.  —  This  is  a  liquid  very  much  like  blood,  but 
without  red  corpuscles.     In  fact,  it  is  chiefly  blood-plasma 
which  has  osmosed  out  through  the  capillaries  into  the  very 
small  spaces  which  exist  between  the  cells  in  all  tissues. 
Thus  lymph  comes  into  direct  contact  with  the  cells,  giving  to 
them  food  and  oxygen,  which  are  dissolved  in  plasma,  and 
receiving    from    the    cells    some    excretions.     These   small 
lymph-spaces  -between  the   cells   are  united  together  into 
larger  lymph-capillaries,   which  in  turn  connect  with  the 
large  lymph-vessels.     These  pour  their  lymph  into  the  veins 


486  APPLIED  BIOLOGY 

in  which  blood  goes  back  to  the  heart.  Lymph,  then,  os- 
moses from  blood-capillaries  into  lymph  spaces,  then  flows 
into  lymph-capillaries,  and  through  larger  vessels  back  into 
the  blood.  The  arrangement  of  blood  and  lymph- vessels 
in  any  organ  might  be  compared  with  the  irrigation  systems 
for  watering  agricultural  land.  The  blood-capillaries  corre- 
spond to  the  ditches  from  which  water  seeps  out  into  the 
spaces  between  particles  of  soil.  These  spaces  in  the  soil 
correspond  to  lymph-spaces,  while  the  lymph-capillaries  and 
larger  tubes  correspond  to  the  drain  tiles  which  carry  excess 
water  away  from  irrigated  soil.  In  short,  the  lymph-spaces 
in  any  organ  constitute  a  sort  of  combined  irrigation  and 
drainage  system  for  cells  which  are  not  directly  reached  by 
the  main  canal  system  of  the  blood-capillaries. 

Lymph  contains  white  corpuscles,  which  are  able  to  squeeze 
through  the  walls  of  blood-capillaries  into  lymph-spaces. 
They  are  also  formed  in  great  numbers  in  the  lymphatic 
glands  through  which  lymph  flows  on  its  way  back  to  the  blood 
in  the  large  veins.  In  fact,  all  the  white  corpuscles  in  blood 
are  lymph-cells  washed  into  the  blood  from  the  lymphatic 
glands.  The  spleen  and  the  tonsils  are  examples  of  large 
lymphatic  glands,  and  there  are  hundreds  of  small  ones  in 
various  parts  of  the  body. 

CIRCULATION  OF  THE  BLOOD 

409.  Need  of  Movement  of  Blood.  —  In  order  that  blood 
may  serve  its  purpose  as  a  distributor  of  food  and  oxygen  to 
the  cells  and  in  removing  excretions  from  the  cells  to  the 
excretory  organs,  it  is  necessary  that  it  should  be  kept  con- 
stantly in  motion.  This  is  accomplished  by  the  circulation 
of  the  blood  from  the  heart  through  the  arteries  and  capil- 
laries into  the  veins,  which  conduct  it  back  to  the  heart. 
That  the  blood  thus  goes  around  in  a  circuit  was  unknown 
until  1621,  when  Dr.  Harvey,  of  England,  proved  that  in 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       487 

man  and  other  vertebrates  the  blood  always  flows  in  one 
direction  from  the  heart  in  blood-vessels,  and  that  these  are 
so  arranged  that  blood  ultimately  comes  back  to  the  heart, 
i.e.,  makes  a  complete  circuit.  This  circulation  is  remark- 
ably rapid;  it  is  believed  that  blood  goes  from  the  heart 
through  the  capillaries  in  such  a  distant  organ  as  a  foot  and 
returns  to  the  heart  in  about  half  a  minute.  (How  many 
times  would  the  blood  complete  its  circuit  in  a  day?) 

410.  Structure  of  the  Heart.  —  (D  or  L)  Procure  from  the  market  a 
sheep's  heart  with  the  lungs  attached,  and  with  the  membrane 
(pericardium)  surrounding  the  heart.  Insert  a  large  tube  in  the 
trachea  and  inflate  the  lungs  by  blowing  into  them.  Note  the  rela- 
tion of  lungs  and  heart.  Carefully  dissect  away  (with  forceps  and 
scissors)  the  fat  which  adheres  to  the  heart,  taking  care  not  to  cut  off 
any  arteries  or  the  thin-walled  veins.  It  is  best  to  stuff  the  veins 
with  cotton  or  insert  a  small  roll  of  paper. 

The  general  form  and  external  structure  of  the  sheep's  heart  is 
similar  to  that  described  and  illustrated  for  the  human  heart  in  many 
books  on  anatomy  and  physiology.  Examine  pictures  in  such  books  ; 
note  positions  of  the  two  auricles  and  two  ventricles  of  the  sheep's 
heart.  Notice  that  a  probe  (e.g.,  a  rounded  stick)  inserted  into  a  vein 
enters  an  auricle.  The  connection  of  the  arteries  with  the  ventricles 
can  be  seen  later  (next  paragraph).  Cut  across  the  heart  transversely 
about  an  inch  from  the  pointed  end  (apex).  This  will  open  the  two 
ventricles.  The  left  one  is  a  rounded  cavity,  the  right  is  crescentic 
in  outline.  Note  the  relative  thickness  of  the  muscular  walls  of  the 
two  ventricles.  Now,  take  a  blunt  stick  about  the  size  of  a  pencil,  and 
inserting  it  into  the  left  ventricle,  probe  carefully  until  it  emerges  out 
of  the  largest  artery.  This  is  the  aorta,  whose  branches  are  arteries 
leading  to  all  the  organs  except  the  lungs.  Insert  a  similar  stick 
into  the  right  ventricle  and  out  through  its  artery.  This  is  the  pul- 
monary artery,  whose  branches  conduct  blood  to  the  lungs. 

The  action  of  the  valves  in  the  aorta  or  pulmonary  artery  in  pre- 
venting blood  from  flowing  back  into  the  ventricles  can  be  demon- 
strated on  a  heart  with  the  apex  removed,  as  follows.  Insert  a  large 
glass  tube  (about  \"  or  f "  caliber)  into  one  of  these  arteries  held 
upright,  and  fill  it  with  water.  Or  connect  the  artery  with  a  large 
funnel.  If  the  valves  are  still  in  good  order,  the  water  will  remain  in 
the  tube  or  funnel. 


488  APPLIED  BIOLOGY 

To  demonstrate  the  action  of  the  auriculo-ventricular  valves, 
cut  away  the  auricles  from  a  heart  with  the  apex  removed,  and  then 
plunge  it  with  the  ventricle  held  downward  into  water  so  as  to  float 
the  valves  into  the  closed  position. 

Slit  open  the  aorta  to  expose  its  valves  (semilunar) ;  and  also  cut 
the  side  of  a  ventricle  to  show  attachment  of  the  auriculo-ventricular 
valves. 


411.  The  cause  of  circulation  is  the  constant,  rhythmic 
beating  of  the  heart,  which  is  a  muscular  force-pump.  Its 
general  plan  of  structure  is  illustrated  by  an  ordinary  rubber 
bulb  such  as  is  used  for  atomizers  and  syringes.  In  such  a 
bulb  there  are  two  valves  arranged  so  that  when  the  bulb 
is  filled  with  water  and  then  compressed,  one  valve  (inlet) 
closes  and  prevents  the  water  from  flowing  outward,  while 
the  other  valve  (outlet)  remains  open  and  allows  the  water 
to  escape  into  the  outlet  tube.  Then  if  the  bulb  be  allowed 
to  expand,  the  outlet  valve  is  closed  by  the  back  pressure  of 
the  water  in  the  outlet  tube,  and  water  rushing  in  from  the 
supply,  tube  opens  the  inlet  valve.  The  next  compression 
will  again  arrange  the  valves  as  first  described  above.  (Draw 
diagrams  to  illustrate  positions  of  valves  and  with  arrows 
show  direction  of  flow  through  a  two-valved  bulb.) 

In  a  similar  manner,  there  are  valves  arranged  in  the  heart 
in  two  places,  one  valve  to  prevent  a  flow  of  the  blood  back 
into  the  veins  (inlet)  when  contraction  occurs,  the  other  to 
prevent  the  return  flow  from  the  arteries  (outlet  tube)  when 
relaxation  or  dilation  takes  place.  And,  as  can  be  easily 
demonstrated  with  a  rubber  bulb  having  one  inlet  and  one 
outlet  valve,  repeated  contraction  and  expansion  will  cause 
fluids  to  flow  through  always  in  the  same  direction,  which  is 
determined  by  the  arrangement  of  the  valves.  Hence,  the 
blood  circulates  because  the  heart  is  a  pump  with  valves  so 
arranged  as  to  force  the  blood  to  flow  in  only  one  direction, 
out  through  one  valve,  and  thence  around  through  tubes 
which  lead  back  to  the  inlet  valve. 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       489 

The  valves  (Fig.  161)  in  all  but  the  largest  veins  also  aid 
in  the  circulation  of  blood,  especially  when  contracting  mus- 
cles press  upon  them  between  two  valves.  The  region  so 
pressed  acts  like  a  syringe-bulb  which  has  outlet  and  inlet 
valves,  each  compression  driving  blood  towards  the  heart. 


H 


FIG.  161.  A,  pocket-like  valves  of  veins.  Arrow  pointing  from  c  to  h  showa 
valves  opening  towards  the  heart,  while  the  lower  figure  shows  how  the 
valve  would  float  up  and  close  if  blood  started  to  flow  backward  towards 
the  capillaries  through  which  it  came. 

412.  Heart  a  Duplex  Pump.  —  Like  some  pumping-ma- 
chines  with  two  pumps  united  side  by  side,  the  heart  of  man 
and  other  mammals  is  double.  To  illustrate  it  correctly 
with  the  rubber  bulb  mentioned  in  above  paragraph,  it  would 
be  necessary  to  place  two  bulbs  side  by  side.  One  bulb 
would  represent  the  right  and  the  other  the  left  ventricle  of 
the  heart.  The  right  ventricle  pumps  blood  into  arteries 
which  extend  to  the  lungs,  and  the  left  pumps  blood  to  all 
the  other  organs.  Moreover,  the  blood  pumped  from  the 
right  side  of  the  heart  to  the  lungs  comes  back  in  veins  which 
connect  with  the  left  side,  and  the  blood  from  the  left  ventricle 
comes  back  to  the  right  side  of  the  heart.  In  other  words,  the 
two  sides  are  constantly  supplying  each  other.  Both  right 
and  left  ventricles  contract  at  the  same  time,  the  right  one 
forcing  blood  into  the  lungs,  and  thence  into  the  left  collect- 
ing chamber  (left  auricle),  while  the  left  ventricle  forces  blood 
into  all  other  organs,  from  which  the  blood  returns  to  the 
right  collecting  chamber  (right  auricle). 

It  is  evident  from  the  above  that  the  two  sides  of  the  heart 


490  APPLIED  BIOLOGY 

must  pump  equal  amounts  of  blood,  for  they  beat  (contract) 
together,  and  each  supplies  the  other  with  blood.  Why  then 
should  the  walls  of  the  right  ventricle  be  so  much  thinner  than 
those  of  the  left?  The  answer  is  that  the  right  ventricle 
pumps  to  the  lungs,  which  are  near  the  heart,  while  the  left 
ventricle  pumps  to  all  the  distant  parts  of  the  body.  It  is 
a  familiar  fact  that  a  force-pump  that  empties  water  into  a 
near-by  bucket  is  easier  to  work  than  one  connected  with  a 
long  line  of  pipe  or  hose. 

413.  Pulse.  —  The  well-known  throbbing  movement  of 
certain  arteries  (as  in  wrist  and  in  temple)  which  are  near 
enough  to  the  skin  to  be  felt  with  the  fingers,  is  due  to  a  rush 
of  blood  through  the  arteries  and  a  sudden  expansion  of 
walls. 

(D)  It  may  be  imitated  with  a  soft  rubber  tube  (5  or  6  feet  long) 
connected  to  a  pump  or  syringe-bulb.  While  pumping  water, 
steadily  pres3  upon  the  tube  with  a  finger,  and  note  the  pulsations 
which  follow  each  stroke  of  the  pump.  Then  insert  a  glass  tube  with 
a  small  opening  into  the  end  of  the  rubber  tube,  and  pump  rapidly 
enough  to  make  the  jet  of  water  a  steady  stream.  Now  note  that 
the  rubber  tube  is  somewhat  expanded  under  internal  pressure,  and 
that  the  "pulse"  is  still  discernible. 

This  simple  experiment  imitates  the  conditions  in  arteries. 
Their  walls  are  elastic,  the  capillaries  exert  resistance  and 
prevent  the  sudden  flow  of  blood  through  them  (just  as 
the  small  opening  at  the  end  of  the  rubber  tube  does) ;  the 
blood  in  the  elastic  arteries  is  then  under  pressure,  and 
the  elasticity  of  the  arteries  steadily  forces  blood  through 
the  capillaries. 

If  the  arteries  were  rigid  like  glass  or  iron,  there  would  be 
no  pulse,  and  blood  would  be  sent  in  sudden  spurts  through 
the  capillaries. 

(D)  To  illustrate  this  replace  rubber  tube  in  experiment  above 
with  a  tube  of  glass  of  same  caliber  and  length.  (Any  desired  length 
may  be  made  by  joining  chemical  glass  tubing  with  short  pieces 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       491 

of  rubber  tubing.)  When  the  pump  is  worked  as  before,  jets  of 
water  will  issue  from  the  end  of  the  glass  tube,  no  matter  how  large 
or  small  an  opening  is  present.  The  same  thing  happens  in  an  ordi- 
nary force-pump,  and  in  order  to  get  a  steady  flow  of  water  in  our 
city  water  systems  an  air-chamber  near  the  pump  is  used.  (Explain 
action  of  air-chamber.)  Obviously  the  elastic  arteries  afford  a  simple 
way  to  get  the  effect  of  an  air-chamber  on  a  force-pump ;  namely, 
a  steady  flow  of  fluid  through  the  small  branches  (capillaries)  of  the 
system  of  tubes. 

In  §  414  it  is  shown  that  another  advantage  of  elastic 
arteries  is  in  regulating  the  flow  of  blood  to  the  organs. 

414.  Regulation  of  Blood  Flow.  —  It  often  happens  that 
an  increased  activity  of  the  organs  demands  a  greater  move- 
ment of  blood  for  carrying  more  food,  oxygen,  or  excretions ; 
or  a  decreased  activity  demands  a  lessened  circulation.  In 
short,  there  is  need  of  a  regulated  blood  flow  from  the  heart 
through  the  organs.  This  is  brought  about  in  two  ways; 
(1)  by  control  of  the  heart,  and  (2)  control  of  the  arteries. 

Control  of  the  heart  is  through  the  nervous  system,  certain 
nerves  carrying  to  the  heart  impulses  or  "  messages  "  which 
increase  the  rate  of  its  beat,  while  other  nerves  inhibit  or 
diminish  the  beat. 

Control  of  the  arteries  is  likewise  the  function  of  certain 
nerves.  The  elastic  small  arteries  in  all  living  organs  are 
usually  somewhat  constricted,  thus  offering  some  resistance 
to  arterial  flow,  and  keeping  up  a  continuous  stream  through 
the  capillaries  and  veins  back  toward  the  heart.  This  con- 
striction or  reduced  caliber  is  produced  by  a  contraction  of  the 
muscle-fibers  which  are  in  the  walls  of  arteries,  and  this  con- 
traction is  under  control  of  nerves. 

When  an  organ  needs  an  increased  blood-supply  (e.g., 
the  stomach  in  digestion),  it  may  be  obtained  by  a  nervous 
reaction  leading  to  a  relaxation  of  the  muscles  of  smaller 
gastric  arteries  and  an  increased  diameter.  Similarly,  the 
application  of  hot  water  or  alcohol  to  the  skin  leads  reflexly 
to  an  increased  blood  flow;  but  cold  water  produces  the 


492  APPLIED  BIOLOGY 

reverse  effect  of  contraction  of  the  arteries  and  a  reduced 
circulation  of  blood  (see  bathing,  §  458).  Also  see  account  of 
heat  regulation  by  the  skin  (§§  447,  453). 

With  the  exception  of  the  heart  itself  and  the  lungs  and 
brain,  the  larger  organs  of  the  body  are  well  supplied  with 
nerves  able  to  regulate  the  size  of  the  smaller  arteries  and 
consequently  the  amount  of  blood-supply. 

Whenever  there  is  a  great  increase  in  caliber  of  the  blood- 
vessels, the  heart's  beat  is  increased  in  order  to  keep  up  the 
pressure.  In  this  way,  certain  drugs  used  by  physicians 
may  increase  the  action  of  the  heart  by  reducing  the  pres- 
sure in  dilated  arteries. 

USE  OF  FOODS  IN  THE  CELLS 

415.  We  have  now  studied  the  preparation  and  distribu- 
tion of  foods  to  the  cells.     Naturally  we  now  ask  questions 
regarding  the  use  which  the  cells  make  of  the  food  supplied 
to  them.     In  studying  the  frog,  we  have  noted  that  foods  are 
used  in  part  for  supplying  energy  and  in  part  for  repair  and 
growth.     In  order  to  apply  this  fact  to  human  life  we  need 
to  consider  more  carefully  the  kinds  of  foods  in  relation  to 
the  supply  of  energy  and  materials  for  repair  and  growth. 
Are  all  foods  equally  valuable?    Should  we  give  any  atten- 
tion to  the  selection  of  our  daily  diet  ?     Such  questions  are 
common  nowadays,  and  show  that  there  is  a  widespread 
popular  interest  in  the  uses  of  various  foods.     We  shall  first 
consider  foods  for  energy  (especially  heat  and  muscular  work), 
and  later,  foods  for  repair  and  growth.     This  is  only  a  con- 
venient division  into  topics,  for  some  foods  can  serve  all 
purposes. 

416.  Energy  and  its  Conservation.  —  Some  general  con- 
siderations regarding  energy  will  make  clear  the  use  of  foods 
as  a  source  of  power  in  the  human  body. 

We  are  familiar  with  the  fact  that  stored  energy  in  coal 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       493 

may  be  changed  into  mechanical  energy  by  a  steam-engine  ; 
and  this  into  electrical  energy  by  a  dynamo ;  and  the  elec- 
trical energy  into  heat  energy  by  an  electric-stove,  or  into 
light  by  an  electric-lamp,  or  into  mechanical  energy  by  a 
motor.  Thus  the  stored  energy  of  coal  may  be  changed  into 
various  other  forms  of  energy  which  are  known  as  mechanical 
energy,  heat,  light,  and  electricity. 

One  of  the  important  discoveries  of  the  last  century  was 
that  in  such  changes  no  energy  is  lost  to  the  world.  At  first 
this  statement  will  be  puzzling  to  one  who  has  read  that 
85  per  cent  of  the  energy  stored  in  a  ton  of  coal  does  not 
appear  in  the  work  of  the  best  steam-engines  in  common  use, 
and  hence  seems  to  be  lost  energy.  Also,  there  appears  to 
be  a  loss  in  transforming  to  electricity,  for  a  20-horse-power 
steam-engine  driving  a  dynamo  cannot  generate  enough 
electricity  to  drive  a  20-horse-power  electric-motor.  How- 
ever, this  loss  of  energy  from  the  practical  standpoint  is, 
after  all,  not  a  real  loss,  for  the  same  amount  of  energy 
remains  in  the  universe.  This  could  be  proved  by  placing 
a  small  steam-boiler  and  its  furnace,  an  engine,  a  dynamo, 
and  a  motor  together  in  a  room  with  walls  which  would 
prevent  loss  of  heat.  Then  if  some  fuel  were  burned  and 
the  machinery  set  to  work,  generating  steam  and  electricity, 
the  explanation  of  the  apparent  loss  of  energy  would  be 
found  chiefly  in  the  heat  given  off  to  the  air  in  the  room. 
Adding  this  to  the  energy  of  the  engine,  dynamo,  and  motor, 
the  sum  would  equal  the  amount  of  energy  which  a  chemist 
can  demonstrate  by  burning  the  same  amount  of  coal  in  a 
calorimeter  (§  418).  In  short,  from  85  to  90  per  cent  of  the 
energy  stored  in  coal  is  given  off  from  the  machinery  as  heat, 
10  to  15  per  cent  appears  in  the  mechanical  energy  of  the 
steam-engine,  some  less  in  the  electrical  energy  of  the  dynamo, 
still  less  in  the  energy  manifested  in  the  electric-lamp,  heater, 
or  motor.  But  each  apparent  loss  is  represented  by  heat 
given  off  to  the  surrounding  air.  The  fact  is,  then,  that 


494  APPLIED  BIOLOGY 

energy  can  be  transformed  from  one  form  into  another 
without  loss,  or  reducing  the  amount  of  energy  in  the  uni- 
verse; that  is,  energy  cannot  be  destroyed.  The  total 
amount  of  energy  in  the  universe  is  always  the  same. 

Another  important  discovery  is  that  energy  is  not  now 
being  created  anywhere.  The  most  wonderful  machines 
can  do  nothing  but  transform  energy.  For  example,  the 
steam-engine  is  a  machine  for  changing  heat  energy  into 
motion  or  kinetic  energy.  A  water-wheel  changes  the  poten- 
tial energy  of  water  stored  at  an  elevation  into  motion.  A 
watch  changes  into  motion  the  energy  stored  in  a  wound-up 
spring.  An  electric-battery  changes  the  energy  stored  in 
chemicals  into  electricity.  And  so  we  might  go  through  the 
whole  list  of  known  physical  changes  in  the  universe,  and  in 
each  case  find  evidence  that  transformation  of  energy,  and 
not  destruction  and  creation,  is  constantly  occurring  in  the 
universe.  Science  has  found  no  positive  evidence  concerning 
either  creation  or  destruction  of  energy  in  all  its  forms. 

The  facts  regarding  changes  of  energy  are  now  embodied 
in  the  Law  of  Conservation  of  Energy,  which  is,  that  energy 
can  be  transformed  from  one  form  into  another,  but  cannot 
be  created  or  destroyed. 

One  of  the  most  interesting  phases  of  science  study  is  that 
of  the  changes  of  energy  as  presented  in  the  science  of  physics. 
The  law  of  conservation  of  energy  has  numerous  practical 
applications  in  machinery.  For  example,  a  physicist  would 
never  spend  his  time  in  trying  to  invent  a  "  perpetual- 
motion  machine,"  for  that  would  be  completely  opposed  to 
the  established  law  of  conservation  of  energy. 

417.  Energy  of  the  Human  Body.  —  This  is  manifested 
externally  in  the  form  of  heat,  and  muscular  and  nervous 
activity.  The  source  of  this  energy  is  the  stored  or  potential 
energy  of  food,  and  numerous  experiments  have  demonstrated 
that  the  law  of  conservation  of  energy  applies  to  the  human 
body  (and  all  other  living  things),  just  as  it  does  to  a  steam- 


HUMAN   STRUCTURE  AND  LIFE-ACTIVITIES       495 

engine.  Food  for  the  human  engine  and  fuel  for  a  steam- 
engine  are  both  necessary  because  they  contain  stored 
energy.  It  is  interesting  to  recall  (§  116)  that  in  both  cases 
the  original  source  of  the  stored  energy  was  sunlight. 

418.  Foods  for  Energy.  —  The  energy  value  of  the  three 
important  nutrients  (proteins,  carbohydrates,  and  fats)  is 
easily  computed  by  chemists.  Samples  of  such  nutrients  are 
dried  and  then  quickly  burned  inside  an  apparatus  known  as 
a  calorimeter  (heat-measurer),  and  the  heat  thus  generated 
is  measured  in  terms  of  the  amount  of  heat  necessary  to  raise 
1000  grams  of  water  one  degree  Centigrade.  This  amount 
of  heat  is  a  calorie,  the  standard  unit  for  heat  measurements  ;* 
and  since  heat  may  be  converted  into  other  forms  of  energy 
(§  416),  it  is  convenient  for  measuring  the  total  energy  of 
foods. 

The  heat  value  of  a  gram  of  food  measured  in  this  way  is 
as  follows  :  for  dry  protein,  about  5.6  calories  (i.e.,  5600  grams 
of  water  1°) ;  for  carbohydrates,  about  4.1 ;  and  for  fats, 
about  9.3.  It  has  been  shown  by  a  method  described  in  the 
next  paragraph  that  a  man  or  a  higher  animal  obtains 
9  calories  from  fats,  and  4  from  carbohydrate  foods;  but 
protein  is  less  completely  oxidized  in  the  living  body,  so  that 
one  gram  of  protein  gives  only  about  4  calories  of  the  5.6  it 
contains.  Obviously  proteins  are  not  economical  as  foods  for 
heat  or  muscular  energy,  while  carbohydrates  and  fats  give 
almost  as  much  energy  in  the  body  as  when  they  are  com- 
pletely burned  in  a  chemist's  calorimeter.  The  chemical 
result  for  fats  and  carbohydrates  is  the  same  in  both  cases, 
for  these  foods  produce  carbon  dioxide  (CO2)  and  water 
(H2O)  in  both  the  living  body  and  the  calorimeter. 

Many  substances  which  will  produce  heat  energy  in  a 


*  Some  authors  use  the  "small  calorie,"  which  is  the  amount  of  heat 
necessary  to  raise  one  gram  of  water  one  degree  C.  It  has  an  advantage  in 
measuring  small  quantities  of  heat  less  than  enough  to  raise  1000  grams  of 
water  one  degree. 


496  APPLIED  BIOLOGY 

calorimeter  could  not  be  burned  in  an  animal  body.     F.ur 
example,  a  gram  of  pure  carbon  (charcoal)  gives  8.08  calories, 
and  a  gram  of  hydrogen  would  give  34.5  calories ;  but  neither 
of  these  could  be  oxidized  at  the  temperature  suitable  :* 
protoplasm.     This   shows    the    importance    of   determin 
whether  foods  give  as  much  heat  in  an  animal  body  as  ir 
calorimeter ;    and  this  hs  3  been  tested  with  a  special  ap£ 
ratus  as  follows  :  — 

419.  Calorimeter  for  Living  Animals.  —  This  consists 
a  small  room  with  walls  arranged  to  prevent  heat  conductk/' 
as  far  as  possible,  with  coils  of  metallic  pipes  arranged  so  that 
circulating  water  will  absorb  heat  from  the  air  in  the  room, 
and  with  machinery  for  ventilation.  Delicate  apparatus 
attached  to  the  pipes  for  supplying  air  and  water  recor^1 
changes  in  temperature,  and  chemical  analyses  are  made  '? 
foods,  breathed  air,  and  excretions.  There  is  also  inside  t1  - 
room  a  foot-power  machine  connected  with  a  small  dynarru 
or  otherwise  arranged  so  that  the  man  may  treadle  thej 
machine  and  thus  do  easy,  moderate,  or  hard  work  for  eight 
hours  per  day.  Thus  it  is  possible  to  determine  the  effect 
of  different  amounts  of  work  upon  food  requirements. 

Inside  such  a  special  calorimeter  men  have  often  lived 
constantly  for  several  days  at  a  time ;  and  chemists  have 
carefully  compared  the  heat  value  of  the  foods  oxidized 
with  the  heat  given  off  from  their  bodies  to  the  air  of  the  room 
and  measured  by  the  apparatus. 

The  results  of  many  such  experiments  have  shown  that 
foods  are  oxidized  as  stated  in  §  418  above.  The  apparatus 
has  proved  of  great  value  in  cases  where  the  value  of  a  food 
to  a  living  animal  or  to  man  was  unknown.  For  example, 
the  dispute  as  to  whether  alcohol  is  a  food  (§  472)  has  been 
settled  by  showing  that  in  very  small  quantities  alcohol 
serves  as  a  food  for  energy ;  and  that  the  amount  so  usable 
is  so  small  as  to  be  of  no  interest  in  the  ordinary  daily  diet 
of  healthy  persons. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       497 

Many  experiments  with  different  combinations  of  foods 
have  led  to  the  conclusion  that  men  of  about  150  pounds 
T     ght  engaged  in  sedentary  work  require  food  yielding  from 
0  to  2400  calories  per  day  of  24  hours.     Men  doing  hard 
ysical  work  for  eight  hours  per  day  require  food  furnishing 
m  3000  to  4000  calories ;  and  very  hard  work  may  require 
>re  than  5000  calories.     There  are  individual  variations 
long  men  of  equal  weight  doing. the  same  work.   Some  idea 
the  meaning  of  these  figures  will  be  gained  from  the  state- 
ment that  100  grams  of  protein  food  giving  400  calories, 
100  grams  of  fat  giving  900  calories,  and  250  grams  of  car- 
bohydrates giving  1000  calories  makes  a  total  of  2300.     This 
would  suffice  for  a  professional  man  or  a  shoemaker,  but  a 
ijrmer,  mason  or  carpenter  would  require    more  fat  and 
.   rbohydrate  food  daily. 

420.  Foods  for  Muscular  Work.  —  It  was  formerly 
upposed  that  protein  foods  are  necessary  for  muscular 
work,  and  we  frequently  hear  that  "  laborers  must  eat  much 
meat  in  order  to  get  strength  to  work."  This  is  not  true. 
It  has  been  shown  that  muscular  exercise  does  not  require 
more  protein  food,  for  fats  or  carbohydrates  can  furnish  the 
necessary  extra  energy.  Two  physiologists  who  used  a 
mixed  diet  of  proteins  and  the  other  foods  have  proved  that 
their  bodies  oxidized  as  much  protein  on  a  day  of  rest  as 
on  a  day  when  they  climbed  a  mountain  over  6000  feet 
high;  and  also  that  the  amount  of  energy  in  the  protein 
they  oxidized  was  not  sufficient  to  lift  their  bodies  so  high. 
Hence  the  other  foods  (fats  and  carbohydrates)  eaten  must 
have  supplied  the  necessary  extra  energy.  But  if  these  men 
had  eaten  more  pure  protein,  they  could  have  gotten  from 
it  the  necessary  energy. 

The  conclusion  from  many  such  experiments  is  that  large 
amounts  of  protein  are  not  needed  for  ordinary  work.  The 
necessary  amount  is  approximately  the  same  for  every  day, 
whether  keeping  quiet  or  working,  and  is  from  60  to  100 

2K 


498  APPLIED  BIOLOGY 

grams  of  protein  for  an  average  150-pound  man.  Most 
well-to-do  people  eat  on  the  average  more  than  100  grams  of 
protein  in  meat  daily,  and  also  they  get  much  additional 
protein  in  milk,  eggs,  bread,  all  vegetables  —  in  fact,  there 
is  some  in  most  common  foods.* 

While  the  amount  of  protein  foods  needed  daily  is  quite 
constant  for  the  ordinary  adult,  the  amount  of  the  other 
foods  should  vary  with  the  amount  of  energy  required.  A 
student  certainly  needs  less  fats  and  carbohydrates  than  a 
laborer  does.  So  long  as  digestion  is  good,  variation  in 
weight  is  a  good  index  to  the  amount  of  such  foods  needed ; 
for  if  taken  in  excess  and  digested,  there  will  likely  be  in- 
creased weight  due  to  food  storage  in  the  fat  tissues  of  the 
body.  Loss  in  weight  by  an  adult  who  daily  eats  as  much 
as  100  grams  of  protein  indicates  a  need  of  more  fats  and 
carbohydrates ;  and  whether  starch,  sugar,  or  fats  should 
be  used  depends  upon  availability,  digestibility,  and  taste 
of  the  individual. 

Growing  animals  and  children  require  a  higher  proportion 
of  protein  (see  §  422)  than  do  adults,  who  alone  have  been 
considered  in  the  above  discussion. 

421.  Foods  for  Heat.  —  As  previously  stated,  birds  and 
mammals  must  maintain  a  constant  body  temperature. 
To  a  large  extent  they  do  this  by  preventing  excessive  loss 
from  the  skin  (§447).  The  production  of  heat  is  chiefly 
the  result  of  muscular  activity.  We  all  know  how  easily 
we  can  keep  warm  by  exercise  on  a  cold  day,  but  how  un- 
comfortably cold  we  often  feel  if  we  simply  eat  our  regular 
meals  and  keep  quiet.  For  this  reason  we  require  more 
clothing  or  covering  when  sleeping,  for  then  only  the  circu- 
latory and  respiratory  muscles  and  chemical  changes  in  the 
digestive  organs  are  the  chief  sources  of  heat  production. 

*  Per  cent  of  protein  in  foods  :  lean  meat  15-22  ;  eggs  15  ;  white  bread 
9  ;  dried  beans  22  ;  potatoes  2  ;  peanuts  25  ;  butter  1  ;  outmeal  16  ;  olive 
oil  0  ;  sugar  0  ;  milk  3  ;  green  corn  3  ;  fish  15-20  ;  apples  0.4  ;  rice  8. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       499 

More  food  is  required  in  cold  weather.  The  non-nitroge- 
nous foods  are  most  economically  increased;  but  whether 
fats  or  carbohydrates  should  be  used  for  increased  heat 
largely  depends  upon  availability  and  digestibility.  The  in- 
habitants of  very  cold  regions  can  easily  get  fat  from  animals, 
while  those  in  temperate  regions  can  store  plant  food,  con- 
taining large  amounts  of  carbohydrates,  for  winter  use. 
When  both  kinds  of  foods  are  available,  there  is  great  indi- 
vidual variation  in  taste  for  and  digestion  of  carbohydrates 
and  fats ;  and  any  desired  combination  of  the  two  may  be 
made  provided  that  we  remember  the  energy  value  of  fats 
as  9  calories,  while  sugars  and  starches  have  4  per  gram. 

422.  Foods  for  Repair  and  Growth  of  Protoplasm.  —  At- 
tention has  several  times  been  directed  to  the  fact  that  liv- 
ing substance  (protoplasm)  is  composed  of  protein.  Also, 
it  has  been  stated  that  animal  cells  can  make  new  protoplasm 
only  from  protein  foods,  while  plant  cells  can  make  protein 
from  the  elements  furnished  by  carbohydrates  and  the 
nitrogen-containing  substances  absorbed  from  the  soil. 
There  is  abundant  evidence  that  in  the  human  body  pro- 
tein food  is  required  for  making  new  protoplasm  in  repair 
and  growth ;  and  that  the  foods  lacking  nitrogen  (i.e.,  car- 
bohydrates and  fats)  cannot  answer  this  purpose. 

The  amount  of  protein  food  required  daily  for  repair  of 
the  protoplasm  in  the  cells  of  an  adult  man  is  believed  to  be 
from  60  to  100  grams  (28  grams  to  an  ounce).  However, 
it  should  be  said  that  some  physiologists  hold  that  100  is 
nearly  twice  the  amount  which  is  absolutely  necessary; 
but  many  conservative  authorities  regard  60  grams  of 
protein  as  too  little  for  a  regular  daily  diet  intended  for 
repair  and  the  maintenance  of  good  health. 

If  200  grams  of  protein  be  eaten  daily,  the  nitrogen  ex- 
cretion will  be  double  that  from  the  usual  100  grams.  This 
indicates  (1)  that  protein  is  easily  disintegrated  in  the  living 
cells,  (2)  that  excess  protein  in  the  food  is  not  stored 


500  APPLIED  BIOLOGY 

for  possible  future  use,  and  (3)  that  if  excess  protein  goes 
to  cells  they  use  the  necessary  amount  for  making  new  pro- 
toplasm and  disintegrate  into  excretions  the  remainder, 
which  may  serve  as  a  source  of  energy.  In  other  words, 
at  least  one-half  of  200  grams  of  protein  might  furnish  heat 
or  muscular  energy  as  other  foods  do. 

423.  Why  limit  Protein  Food?  —  Since  protein  is  neces- 
sary for  repair  and  growth,  and  may  also  supply  energy,  we 
may  properly  ask  why  physiologists  so  often  recommend  a 
limited  protein  diet,  e.g.,  60  grams  per  day.  There  are 
several  answers :  — 

(1)  Protein  as  a  source  of  energy  is  physiologically  waste- 
ful, for  the  human  body  can  obtain  only  about  4   calories 
per   gram,  which   in   the   chemist's   calorimeter   shows   5.6 
calories.     This  is  because  nitrogen  excretion  leaves  the  body 
incompletely  oxidized. 

(2)  Protein  for  energy-supply  forces  the  body  to  handle 
the  useless  nitrogen  which  it  contains,  for  energy  comes  from 
oxidation  of  the  carbon  and  hydrogen.     A  hard-working  man 
getting  about  2880  calories  from  a  daily  diet  of  120  grams  of 
protein,  100  of  fat,  and  375  of  carbohydrates  would  have  to 
eat  about  720  grams  of  protein  daily  to  get  the  same  amount 
of  energy  if  he  took  food  other  than  clear  lean  meat.      This 
would  require  the  kidneys  to  excrete   200  extra  grams  of 
urea  which  would  come  from  the  nitrogen  content  of  the 
extra  600  grams  of  protein.     Such  an  excess  of  nitrogen 
excretions  is  injurious  and  tends  to  cause  disease  of  kidneys 
and  other  organs.     Hence  it  is  better  to  get  the  necessary 
energy  from  100  grams  of  fats  and  375  of  carbohydrates 
(100  X  9  +  375  X  4  =  2400  calories)  rather  than  from   600  of 
extra  protein  (600  X  4  =  2400  calories). 

(3)  Protein  is  not  economical  in  a  pecuniary  sense.     It 
costs  much  more   than   common    carbohydrates  (starch  in 
vegetables,  or  even  sugar)  which  have  the  same  energy  value, 
namely,  4  calories  per  gram ;  and  clear  lean  meat  is  much  more 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       501 

expensive  than  fat  meat  or  butter  when  we  consider  that 
fat  has  9  calories  per  gram,  and  hence  has  more  than  double 
the  energy  value  of  protein,  which  has  4  calories  per  gram. 

424.  Mixed  Diet.  — The  three  reasons  given  above  have 
led  all  physiologists  to  advocate  a  mixed  daily  diet  containing 
(1)  the  protein  necessary  for  daily  repair,  and  for  growth  in 
early  life ;  and  (2)  enough  non-nitrogenous  foods  to  furnish 
the  necessary  heat  and  muscular  energy.  Whether  these 
foods  for  energy  should  be  fat  or  carbohydrates  depends  upon 
availability  and  digestibility.  One  man  may  digest  and 
use  100  grams  of  fat  and  300  of  starch  as  well  as  another  could 
use  50  grams  of  fat  and  412  of  starch  ;  but  the  energy  obtained 
is  nearly  the  same. 

Special  foods  are  not  known  for  special  organs.  That 
celery  is  a  "  nerve  "  food,  fish  a  "  brain  "  food,  lean  meat  a 
"  muscle  "  food,  etc.,  are  popular  beliefs  which  are  entirely 
without  scientific  foundation. 

It  is  not  yet  known  whether  proteins  from  plants  are 
equally  valuable  with  those  from  meat.  Certainly  many 
people  have  lived  well  on  a  vegetarian  diet ;  but  those  who 
adopt  it  in  its  strictest  form  without  milk,  cheese,  and  eggs 
should  eat  more  than  60  grams  of  protein,  because  it  is 
probable  that  the  human  cells  select  only  certain  kinds  of 
plant  proteins  for  use  in  repair  and  growth  of  protoplasm. 

It  also  should  be  noted  that  with  a  strict  vegetarian  diet 
there  may  be  difficulty  in  getting  enough  protein  without 
an  excess  of  carbohydrates,  because  there  is  relatively  little 
protein  in  many  plant  tissues.  However,  certain  seeds  (es- 
pecially beans,  peas,  'lentils,  wheat,  rye,  and  oats)  contain 
much  more  protein  than  the  foods  obtained  from"  green  vege- 
tables "  (roots,  stems,  and  leaves  of  plants).  Hence,  it  is 
possible  by  using  such  seeds  to  make  a  strict  vegetarian  diet 
with  proper  proportions  of  protein  and  other  food.  How- 
ever, the  safest  way  for  most  people  is  to  add  milk  and  its 
products  and  eggs  to  plant  foods,  if  one  has  tastes  or  principles 


502  APPLIED  BIOLOGY 

opposed  to  the  use  of  meats.  On  scientific  grounds  there  is 
no  known  objection  to  the  proper  use  of  meats,  but  simply 
an  objection  to  meats  in  excess  of  actual  protein  requirements 
(i.e.,  above  60  to  100  grams  a  day). 

References :  Those  who  are  interested  in  questions  of 
diet  should  read  the  chapters  on  "  Nutrition,"  and  "  Hygiene 
of  Feeding  "  in  Hough  and  Sedgwick's  "  Human  Mechan- 
ism." Also  obtain  from  the  Department  of  Agriculture, 
at  Washington,  the  bulletins  on  the  nutritive  values  of 
foods. 

OXYGEN-SUPPLY 

426.  Respiration.  —  This  has  already  been  defined  as 
including  the  functions  of  obtaining  oxygen  and  eliminating 
carbon  dioxide.  In  some  lower  animals  (e.g.,  earthworm) 
the  skin  is  the  respiratory  organ ;  in  fishes  and  others  there 
are  gills;  amphibians  breathe  with  both  skin  and  lungs; 
but  in  the  vertebrates  higher  than  the  amphibians  lungs 
are  the  sole  respiratory  organs.  In  all  these  cases  the  mem- 
branes which  take  up  oxygen  also  give  out  or  excrete  carbon 
dioxide.  For  greater  convenience  in  study,  we  shall  in  this 
lesson  confine  our  attention  to  the  supplying  of  oxygen  to 
cells  in  the  human  body;  and  deal  with  the  excretion  of 
carbon  dioxide  in  the  next  lesson. 

The  respiratory  organs  consist  of  nasal  passages,  pharynx, 
larynx,  trachea  (windpipe),  bronchi  (right  and  left  branches 
of  the  trachea),  bronchial  tubes  (branches  of  bronchi),  air- 
chambers  at  ends  of  smallest  bronchial  tubes,  diaphragm, 
and  wall  of  the  thorax-  or  chest-cavity. 

426.  Respiratory  Passages.  —  We  commonly  think  of 
the  nose  as  an  organ  for  the  sense  of  smell ;  but  the  fact  is 
that  only  a  limited  amount  of  epithelium  (lining  membrane) 
in  some  of  the  upper  nasal  cavities  has  nerve-endings  con- 
nected with  the  olfactory  part  of  the  brain,  and  hence  has 
the  power  of  perceiving  odors.  Most  of  the  cavities  in  the 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       503 

nose  are  respiratory,  and  serve  the  purpose  of  warming  the 
inrushing  air  by  contact  with  the  warm  membranes;  and 
also  the  membranes  and  hairs  in  the  nose  collect  much  of 
the  dust  from  the  air  which  enters. 

The  passages  connecting  those  of  the  nose  with  the  pharynx 
are  called  post-nasal.  They  lie  back  of  the  soft  palate. 
Into  the  same  passages  open  the  Eustachian  tubes  from  the 
ears.  Some  of  the  soft  tissues  in  the  lining  of  the  post- 
nasal  chamber  may  enlarge  and  form  adenoids  (§449),  which 
interfere  with  free  breathing  through  the  nose  and  cause 
injurious  breathing  through  the  mouth. 

427.  Lungs/—  (D  or  L)  Examine  sheep's  lungs  obtained  from  a 
meat-market.  Notice  the  trachea  and  the  arrangement  of  its  carti- 
laginous rings,  incomplete  on  the  side  next  to  the  esophagus.  Follow 
the  two  branches  (bronchi)  to  the  lungs.  Try  inflating  the  lungs  by 
forcing  air  through  a  large  glass  or  wooden  tube  inserted  into  the 
trachea.  Why  do  the  lungs  collapse  and  force  out  air  when  pressure  is 
released  ?  Compare  action  with  that  of  a  rubber  bag  or  balloon.  Note 
that  there  is  air  remaining  in  all  parts  of  the  lungs  after  their  complete 
return  to  the  original  size  before  inflation.  In  human  lungs  after 
ordinary  breathing  out  or  expiration  of  air,  the  amount  of  air  still 
left  is  about  200  cubic  inches,  and  one-half  of  this  can  be  forced  out 
by  "blowing  hard"  (i.e.,  forcibly  expiring  air,  which  is  accomplished 
by  muscular  compression  of  the  lungs).  This  additional  discharge 
of  air  from  sheep's  lungs  can  be  demonstrated  by  pressing  upon  them 
with  the  hands.  Of  course,  the  lungs  inside  an  animal  cannot  be 
inflated  larger  than  the  chest-cavity. 

The  delicate  outer  membrane  of  the  lungs  is  the  pleura,  and 
similar  tissue  lines  the  chest-cavity.  These  membranes  are  lubri- 
cated by  a  secreted  liquid  which  reduces  friction  between  the  lungs 
and  the  chest-wall  during  the  movements  of  respiration.  Inflam- 
mation of  these  membranes,  which  often  occurs  in  connection  with 
severe  "colds,"  is  termed  pleurisy. 

Cut  into  one  bronchus  and  then  split  it  open  as  you  work  toward 
the  smaller  branches.  In  this  way  follow  up  and  lay  open  a  series 
of  successively  smaller  branches  leading  out  toward  the  surface  of 
the  lung.  The  smallest  branches  end  in  air-chambers.  Notice  the 
numerous  blood-vessels  in  the  tissues  of  the  lungs.  The  arteries 
can  be  distinguished  by  having  thicker  walls  than  the  veins. 


504  APPLIED  BIOLOGY 

428.  Breathing  Movements.  —  Expansion  of  the  chest- 
cavity  will  result  in  reduced  pressure  so  that  the  external 
air  will  enter  through  the  trachea  until  the  pressure  in  the 
air-chambers  of  the  lungs  balances  that  between  the  lungs 
and  walls  of  the  chest-cavity.  This  is  exactly  what  happens 
in  the  following  experiment. 

(D)  Action  of  Diaphragm.  Use  apparatus  constructed  as  follows  : 
A  sheet  of  dental  rubber  is  stretched  and  tied  over  the  base  of  an 
open-top  bell-jar,  or  lamp-chimney  that  is  wide  at  base  and  narrow 
at  top.  A  short  glass  tube  is  tied  into  the  mouth  of  a  delicate  rubber 
bag  (e.g.,  toy  balloon),  and  the  free  end  of  this  tube  is  then  inserted 
through  a  hole  in  a  cork  which  will  fit  tightly  into  the  top  of  the 
bell-jar.  When  the  cork  is  placed,  the  rubber  bag  should  hang  near 
the  center  of  the  jar.  The  glass  jar  represents  the  walls  of  the  chest- 
cavity,  the  glass  tube  represents  trachea,  the  rubber  bag  stands  for 
elastic  lungs,  and  the  rubber  sheet  at  the  bottom  acts  as  a  dia- 
phragm. Note  that  when  this  is  made  convex  by  pressure,  the  rub- 
ber bag  (imitating  lungs)  collapses.  Why  ?  Why  is  air  forced  out  of 
any  toy  balloon  when  free  to  escape  ?  When  the  rubber  diaphragm 
becomes  flat  (imitating  the  downward  or  posterior  movement  of  the 
human  diaphragm),  the  rubber  "lungs"  expand.  Why?  Why 
does  water  rush  into  a  pump  when  the  piston  is  raised  ?  This  ap- 
paratus illustrates  the  mode  of  respiratory  action ;  but  is  not  exact, 
for  the  lungs  fit  and  fill  the  chest-cavity,  except  that  the  heart  lies  in 
a  space  between  them. 

The  expansion  of  the  human  chest-cavity  is  due  to  breathing 
movements,  which  are  of  two  kinds :  (1)  those  of  the  dia- 
phragm, whose  positions,  due  to  muscular  movements,  may 
be  imitated  by  the  rubber  sheet  across  the  mouth  of  the 
jar  used  in  the  preceding  demonstration  ;  and  (2)  to  expansion 
of  the  side-walls,  increasing  the  diameter  in  a  horizontal 
plane.  This  latter  is  due  to  raising  the  ribs,  and  can  be 
demonstrated  by  measuring  with  a  tape  the  circumference 
of  the  chest  before  and  after  inspiration.  Young  children 
expand  the  chest-cavity  chiefly  by  the  movements  of  the 
diaphragm;  in  many  men  the  diaphragm  does  most  of 
the  breathing  work ;  in  some  women,  particularly  when  tight 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES        505 

clothing  is  worn,  the  diaphragm  is  but  little  used,  and  ex- 
pansion of  the  chest-cavity  is  due  chiefly  to  movements  of 
the  ribs.  The  ideal  breathing  movements  combine  to  use 
both  ribs  and  diaphragm.  Note  effect  of  forced  breathing, 
or  after  rapid  exercise. 

429.  Changes  in  Breathed  Air.  —  Analysis  of  air  which 
has  been  taken  into  the  lungs  and  then  expired  shows  that 
it  has  lost  oxygen.     This  means  that  the  blood  circulating 
in  the  capillaries  of  the  lungs  has  absorbed  some  oxygen  from 
the  air  in  the  air-chambers  around  which  the  blood-capillaries 
are  arranged. 

The  average  adult  person  can  expel  from  his  lungs  after 
the  deepest  possible  inspiration  about  230  cubic  inches  of 
air.  In  ordinary  breathing  a  man  takes  in  about  30  cubic 
inches,  and  expels  about  the  same  amount.  An  additional 
100  (total  130)  cubic  inches  can  be  taken  in  by  forced  in- 
spiration ("  a  long  breath  "),  and  this  much  plus  about 
100  not  usually  expired  (total  230)  can  be  expelled  by  forced 
expiration,  as  stated  above. 

After  the  deepest  possible  expiration  of  230  cubic  inches  of 
air,  about  100  cubic  inches  of  air  remain  in  the  lungs.  It  is 
obvious  that  under  ordinary  breathing  conditions  the  amount 
of  air  inspired  (30  cubic  inches)  is  not  sufficient  to  fill  the 
lungs,  which  contain  130  cubic  inches,  for  it  is  not  quite  one- 
third  of  the  amount  left  in  the  lungs.  The  fact  is  that  the 
100  cubic  inches  of  air  usually  left  in  the  lungs  is  mixed  and 
diluted  with  the  30  of  fresh  air. 

430.  Absorption  of    Oxygen  by  Blood  in  Lungs.  —  The 
blood-supply  to  the  lungs  is  venous,  from  the  right  side  of 
the  heart,  which  in  turn  receives  it  from  all  the  organs  except 
the   lungs.     This   blood   passes   through   the   capillaries   of 
the  lungs  and  leaves  by  way  of  the  pulmonar}'  veins  to  the 
left  side  of  the  heart,  thence  to  all  the  organs  except  the  lungs. 
On  the  way  through  the  lungs  the  blood  is  changed  from  venous 
to  arterial,  by  losing  part  of  the  carbon  dioxide  (§  433)  and 


506  APPLIED  BIOLOGY 

absorbing  oxygen  from  air  in  the  air-chambers  through  the 
delicate  walls  of  the  chambers  and  into  adjoining  blood- 
capillaries.  >*;, 

We  must  guard  against  the  common  error  of  supposing 
that  venous  blood  coming  back  to  the  lungs  from  all  other 
organs  is  devoid  of  oxygen.  The  fact  is  that  every  100  cc. 
of  venous  blood  coming  to  the  lungs  has  about  10  cc.  of 
oxygen  dissolved  in  the  blood,  and  when  it  leaves  as  arterial 
blood  it  has  about  20  cc.  In  other  words,  the  amount  of 
oxygen  is  doubled  in  arterial  blood.  This  additional  oxygen 
becomes  combined  with  the  haemoglobin  in  the  red  cor- 
puscles, and  results  in  the  change  of  color.  Blood  with- 
out the  red  cells  could  not  contain  so  much  oxygen,  and 
hence  vertebrates  have  a  great  advantage  over  the  lower 
animals. 

431.  Distribution  of  Oxygen  to  Cells.  —  Blood  carrying 
oxygen  received  in  the  lungs  passes  from  the  left  auricle  to 
the  left  ventricle  of  the  heart,  and  thence  to  all  the  organs  ex- 
cept the  lungs.  As  the  blood  flows  through  the  capillaries  in 
organs,  some  of  its  oxygen  is  absorbed  by  near-by  cells  and 
some  by  the  lymph,  which  distributes  it  to  cells  that  are 
distant  from  blood-capillaries.  Recall  the  statement  that  all 
living  cells  are  continually  using  oxygen,  and  it  is  obvious 
that  they  as  constantly  must  demand  a  fresh  supply.  If 
the  blood  were  to  stop  flowing  for  even  a  short  time,  the 
available  oxygen  in  the  lymph  and  blood  in  every  organ 
would  soon  be  used  by  the  oxidation  going  on  in  protoplasm  ; 
and  the  result  would  be  asphyxiation  of  the  cells.  But  under 
normal  conditions  the  flow  of  blood  is  so  rapid  that  only 
approximately  one-half  of  its  contained  oxygen  is  extracted 
while  it  flows  from  the  arteries  through  the  capillaries  into 
the  veins.  ;  Hence  venous  blood  returning  to  the  lungs  has 
about  one-half  the  oxygen  of  the  arterial  blood  leaving  the 
lungs. 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       507 


EXCRETION 

432.  Purpose  of  Excretion.  —  The  products  of  oxidation 
in  the  cells  of  the  human  body  are  of  no  further  use,  and  when 
accumulated  may  be  injurious.     Hence  special  organs  have 
as  their  work  the  elimination  of  excretions  (1)  from  the 
blood  and  lymph  which  absorbs  them  from  the  cells,  and 
(2)  from  the  body.     Most  important  of  the  excretions  are, 
as  in  the  case  of  all  other  organisms  we  have  studied,  carbon 
dioxide  (C02),  water  (H20),  and  nitrogenous  excretions.     The 
carbon  dioxide  is  chiefly  excreted  by  the  lungs,  the  water 
by  the  kidneys  (with  some  unnecessary  help  by  the  skin), 
the  nitrogenous  excretions  chiefly  by  the  kidneys. 

433.  Excretion  of  Carbon  Dioxide.  —  Venous  blood  enter- 
ing the  lungs  contains  about  46  cc.   of  carbon  dioxide  in 
100  cc.  of  blood,  and  the  arterial  blood  leaving  the  lungs 
has  only  about  40  cc.     Thus  it  appears  that  approximately 
one-eighth  of  the  contained  carbon  dioxide  is  excreted  as 
blood  flows  through  the  capillaries  in  the  lungs. 

The  chief  difference  between  arterial  and  venous  blood  is 
that  the  arterial  has  twice  as  much  oxygen  and  seven-eighths 
as  much  carbon  dioxide.  The  small  amount  of  nitrogen 
dissolved  in  the  blood  is  always  the  same  (1  to  2  per  cent) ,  for 
free  nitrogen  takes  no  part  in  the  activities  of  living  cells  in 
animals. 

Evidently  it  is  incorrect  to  state  that  "  venous  blood  is 
purified  in  the  lungs  "  and  that  "  arterial  blood  is  pure." 
The  loss  of  only  one-eighth  of  the  carbon  dioxide  is  not  making 
"  pure."  We  should  not  say  that  we  have  made  muddy 
water  "  pure  "  if  only  one-eighth  of  the  mud  is  extracted. 
The  words  "pure"  and  "  purify  "  should  never  be  used  in 
connection  with  respiration  of  blood  in  the  lungs.  It  is 
easy  to  remember  that  blood  doubles  its  oxygen  and  loses 
about  one-eighth  of  its  carbon  dioxide  in  changing  from 
venous  to  arterial  blood  in  the  lungs. 


508  APPLIED  BIOLOGY 

(D)  Carbon  Dioxide  in  Expired  Air.  —  Connect  a  bellows,  bicycle- 
pump,  or  atomizer  bulb  to  a  glass  tube,  and  blow  air  into  some  lime- 
water  (or  barium-water).  Note  the  effect.  If  much  cloudiness 
(precipitate)  appears,  compare  with  air  pumped  out-of-doors  directly 
into  the  lime-water. 

Now,  exhale  air  from  the  lungs  through  a  glass  tube  into  lime- 
water.  Compare  with  the  lime-water  mixed  with  fresh  air  in  above 
experiment. 

434.  Excretion  of  Nitrogen  and  Water.  —  This  work  of 
the  kidneys  is  carried  on  by  the  tubules  which  are  numerous 
in  these  organs.     A  section  of  a  kidney  prepared  for  micro- 
scopic study  shows  tubules  cut  transversely,  longitudinally, 
and  obliquely;    and  surrounding  the  tubules  are  abundant 
blood-capillaries.     The  cells  composing  each  tubule  extract 
nitrogenous  excretions  and  water  from  the  blood,  and  then 
eliminate  them  into  the  cavity  or  lumen  of  the  tubule.     The 
water  washes  the  excretion  out  of  the  tubule  into  the  ureter  or 
kidney-duct,  which  in  all  mammals  extends  from  each  kidney 
to  the  bladder.     This  is  a  reservoir  for  the  temporary  storage 
of  the  excretions  (urine)  of  the  kidneys,  and  in  all  mammals 
is  connected  with  the  exterior  by  a  duct  known  as  the  urethra. 

435.  Skin  in  Excretion.  —  Under  conditions  of  high  tem- 
perature the  human  skin-glands  eliminate  water  ("  sweat  " 
or  "  perspiration  ")  in  which    is   dissolved  salt   and  small 
quantities  of  other  substances  commonly  excreted  by  the 
kidneys.     However,    all    the    water    and    other    substances 
could  be  excreted  by  the  kidneys,  and  so  the  skin  is  not  a 
necessary  organ  of  excretion.     It  will  be  explained  in  §  447 
that  the  skin  eliminates  water  in  order  to  get  rid  of  excess 
heat,  and  hence  excretion  of  water  is  not  the  primary  work 
of  the  skin-glands. 

SUMMARY 

436.  Functions    Serving   the    Cells.  —  (1)  Digestion    and 
absorption  of  foods  are  functions  for  getting  dissolved  foods 
into  the  blood,  which  directly  or  through  the  lymph  distributes 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       509 

it  to  the  cells  of  the  entire  body.  (2)  The  lungs  are  adapted 
to  supplying  the  blood  with  oxygen,  which  is  then  distrib- 
uted to  the  cells.  (3)  The  excretory  organs  (chiefly  lungs 
and  kidneys)  are  adapted  to  remove  from  the  blood  the 
various  excretions  found  in  the  cells.  (4)  The  circulating 
liquids  (blood  and  lymph)  serve  the  functions  of  food-supply, 
oxygen-supply,  and  excretion,  in  that  food  and  oxygen 
must  be  distributed  to  the  cells  and  excretions  taken  away 
to  the  excretory  organs. 

These  four  great  functions,  involving  the  organs  of  digestion, 
respiration,  excretion,  and  circulation,  serve  the  cells.  We 
have  noted  (§  268)  that  a  one-celled  organism  does  not  need 
such  a  complicated  mechanism,  for  with  respect  to  food, 
oxygen,  and  excretions  it  can  deal  directly  with  the  external 
world.  The  vast  number  of  cells  in  a  higher  animal  has 
made  necessary  the  complicated  organs  of  digestion,  respira- 
tion, excretion,  and  circulation  for  serving  the  cells. 

NERVOUS   ACTIVITY 

437.  Need  of  Coordination.  —  The  functions  which  serve 
the  cells  named  in  the  preceding  paragraph  must  work  to- 
gether or  in  harmony.  For  instance,  if  muscles  are  working 
faster,  there  is  need  of  more  food  and  oxygen.  This  requires 
more  rapid  circulation,  greater  oxygen-supply,  and  increased 
digestion  in  order  to  supply  and  transport  the  necessary 
food  and  oxygen.  And  faster  work  results  in  more  excretions 
and  consequently  greater  activity  of  the  circulatory  and  ex- 
cretory organs  in  removing  them.  Thus  increased  activity 
of  certain  organs  may  demand  a  corresponding  increase  in 
work  of  many  others ;  and  there  must  be  coordinated  activity. 

The  function  of  coordination  is  part  of  the  work  of  the 
nervous  system.  This  is  parallel  with  the  case  of  a  frog's 
functions  (§  54) ;  but  also  the  human  nervous  system  has  a 
vast  amount  of  other  work  arising  from  mental  activities. 


510  APPLIED  BIOLOGY 

438.  Reflex  Action.  —  How  the  nervous  system  exerts 
its  coodinating  power  on  other  organs  may  be  clearer  after 
a  brief  account  of  some  simple  cases  of  control. 

If  by  accident  one  touches  a  finger  to  a  hot  stove,  a  sudden 
contraction  of  the  muscles  will  cause  the  hand  to  be  jerked 
away  before  the  brain  becomes  conscious  of  the  burn.  The 
explanation  of  such  an  action  is  that  the  hot  stove  stimulated 
sensory  nerve-endings  in  the  finger,  the  stimulus  was  trans- 
mitted to  nerve-cells  in  the  spinal  cord  between  the  shoulders, 
and  at  once  turned  back  as  a  motor  impulse,  which,  trans- 
mitted along  other  nerve-fibers  back  to  the  finger,  caused 
the  muscles  to  contract.  Since  the  impulse  originating 
in  the  stimulated  sensory  nerve-endings  appears  to  be  re- 
flected back  to  the  muscles  by  the  spinal  cord,  the  process  is 
called  reflex  action. 

Headless  frogs  and  other  animals  will  make  the  same 
reflex  movements  if  their  toes  are  stimulated.  This  proves 
that  reflex  action  is  quite  independent  of  consciousness 
(knowing,  feeling),  for  all  observations  on  injured  men  and 
animals  indicate  that  the  brain  is  the  organ  of  consciousness. 
For  example,  a  man  with  the  spinal  cord  seriously  injured, 
say  in  the  middle  of  the  back,  would  feel  no  pain  in  the  legs 
and  could  not  move  them  voluntarily ;  that  is,  by  conscious 
action  from  the  brain.  Such  facts,  of  which  many  have  been 
recorded  in  medical  books,  prove  that  there  must  be  uninjured 
nerve-fibers  extending  to  the  brain  in  order  to  have  conscious 
control  of  organs.  Hence,  a  headless  animal  or  one  with  the 
spinal  cord  cut  off  just  back  of  the  head  could  not  feel  or 
be  conscious  of  changes  occurring  in  the  body. 

In  the  case  of  instantly  closing  an  eyelid  to  escape  a  threat- 
ened injury,  the  stimulation  of  the  nerve  of  sight  (optic  nerve) 
causes  an  unconscious  reflex  in  the  brain  back  to  muscles 
which  move  the  eyelids.  There  are  many  other  possible 
reflexes  through  nerves  in  the  head  which  are  directly  con- 
nected with  the  brain. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       511 

The  above  are  simple  examples  of  unconscious  reflex 
actions  which  are  constantly  occurring.  We  learn  to  do  a 
large  number  of  things  reflexly,  in  addition  to  the  funda- 
mental processes  such  as  breathing,  heart-beat,  digestion, 
etc.,  which  naturally  and  necessarily  are  subject  to  reflex 
control.  In  walking,  playing  musical  instruments,  using 
various  tools,  etc.,  we  learn  by  long  practice  to  act  more  or 
less  reflexly  or  automatically,  and  with  little  or  no  exercise 
of  the  will  (conscious  control). 

439.  Conscious    or  Voluntary  Action.  —  Touching*  a  hot 
stove  used  above  as  an  illustration  of  reflex   action   also 
affords  a  case  of  conscious  action.     Soon  after  the  hand  is 
jerked  away  by  reflex  action,  one  becomes  conscious  of  being 
burned.     Obviously,  there  are  nerve-fibers  for  transmitting 
the  sensory  impulse  from  the  tip  of  the  burned  finger  to  the 
brain.     There  a  conscious  action  may  occur,  and  it  may  be 
reasoned  that  it  is  dangerous  to  keep  the  hand  anywhere 
near  a  hot  stove,  and  so  on,  with  the  result  that  it  is  decided 
or  willed  to  take  the  hand  far  away.     This  is  accomplished 
by  a  conscious  motor  impulse  from  the  brain  to  the  muscles 
of  the  arm,  causing  them  to  move  as  the  will  dictates. 

Moreover,  the  brain  may  also  control  the  direction  of  the 
movement;  that  is,  it  may  coordinate  the  contraction  of 
the  various  muscles.  A  splendid  example  of  such  coordinated 
voluntary  or  conscious  control  of  muscles  is  that  of  baseball 
pitchers  who  can  will  to  throw  a  ball  to  a  given  place  and 
at  the  same  time  consciously  control  the  contraction  of  the 
muscles  of  the  arm  so  that  the  ball  will  be  given  the  twirling 
or  curving  motion  so  much  desired  by  experts  in  ball-playing. 
Learning  to  do  this  by  long  practice  means  training  the  nerve- 
cells  in  the  brain  so  that  they  will  come  to  control  the  muscu- 
lar contractions  and  so  cause  the  desired  muscular  movements. 

440.  Spinal  Cord  and  its  Nerves.  —  The  human  spinal  cord 
is  usually  described  as  lying  in  a  cavity  in  the  backbone 
(vertebral  column).     Examination  of  some  of  the  segments  of 


512  APPLIED  BIOLOGY 

any  backbone  which  may  be  obtained  at  a  meat-market 
will  show  that  the  cord  lies  dorsal  to  the  central  axis.  More- 
over, the  cord  is  not  completely  covered  by  bone,  and  at 
the  uncovered  places  are  the  spinal  nerves.  These  are  ar- 
ranged in  pairs  (thirty-one  pairs  altogether).  Each  nerve 
is  divided  near  the  cord,  and  one  branch  or  root  joins 
the  dorsal  side  of  the  cord,  while  the  other  joins  the  ven- 

tral  side.  On  each 
dorsal  root  is  a 
thickening  called 
spinal  ganglion, 
which  contains 
nerve-cells  whose 

FIG.  162.     Diagram  showing  relations  of  a  cross  . 

section  of  spinal  cord  to  spinal  nerves.     The  fibers      extend     into 

X-shaped  central   mass  is   gray  matter,   sur-  the  COrd  and  also  out 

rounded  by  white  matter.    PF,  posterior  or  dor-  ,             ,        , 

sal  fissure  ;  AF,  anterior  or  ventral  fissure  ;  PR,  through    the   nerVCS 

dorsal  root ;  AR,  ventral  root  of  spinal  nerve  (Fig   162)      The  Cells 

fe);0»,  spinal  ganglion.     (From  Huxley-Lee.)  ^^    fibers  ^^ 

tute  the  ventral  root  lie  inside  the  cord. 

A  cross  section  of  a  spinal  cord  shows  an  X-  or  H-shaped 
figure  in  the  center.  In  a  fresh  cord  this  is  silvery  gray, 
and  is  called  the  gray  matter  (Fig.  162).  The  surrounding 
whitish  tissue  (white  matter)  is  chiefly  composed  of  con- 
nective tissue  and  covered  nerve-fibers.  Many  of  these 
fibers  extend  to  the  brain  and  others  to  the  regions  of  the 
cord  where  other  spinal  nerves  are  attached.  The  gray 
matter  contains  many  nerve-cells,  especially  those  whose 
fibers  extend  out  through  the  ventral  root  into  the  spinal 
nerve  and  thence  to  muscles  and  other  organs. 

The  spinal  cord  is  surrounded  by  membranes  which  sepa- 
rate it  from  the  hard  walls  of  the  backbone.  These  con- 
tain numerous  blood-vessels,  some  of  which  extend  into  the 
cord.  Similar  membranes  are  between  the  brain  and  the 
skull. 

Deep  furrows  or  fissures  extend  longitudinally  on  both 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       513 


514 


APPLIED  BIOLOGY 


spinal  cord  with  the  brain.  Most  of  these  structures  can 
be  identified  on  a  sheep's  brain,  obtained  from  a  meat-market, 
and  hardened  in  strong  alcohol  or  formalin  solution. 

Microscopic  study  of 
sections  shows  that  most 
of  the  nerve-cells  of  the 
brain  are  near  the 
surface  (cortex),  while  in- 
side are  the  fibers  con- 
necting with  the  spinal 
cord  and  also  connecting 
various  parts  of  the  brain. 
Hence  a  cut  across  a  fresh 
brain  shows  gray  matter 
(with  nerve-cells)  on  the 
outside  and  white  matter 
(nerve-fibers)  inside,  just 
the  reverse  of  the  spinal 
cord. 

The  cerebral  hemi- 
spheres are  the  center  of 
mental  life,  conscious- 
ness, and  voluntary 
action.  The  cerebellum 
is  the  center  of  coordina- 
tion, causing  muscles  to 
work  in  definite  and  con- 
trolled ways.  The  nerve- 
cells  which  control  the 
respiratory  movements 
and  the  circulatory  or- 


FIG.  163.  Diagram  showing  nerve  con- 
nections of  ear,  eye,  lips,  and  hand  with 
centers  in  the  cerebrum  of  the  brain. 
The  ear  and  eye  have  only  sensory  fibers 
which  carry  impulses  to  the  brain,  while 
the  lips  and  hand  have  both  sensory  and 
motor  fibers  (m,  s)  carrying  impulses  as 


shown  by  the  arrows. 
and  Stirling.) 


(From  Landois 


are   in   the   spinal 


gans 
bulb. 

Localized  centers  in  the   cerebrum   are  known  to  exist. 
So  far  our  knowledge  is  limited,  but  there  are  areas  in  the 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       515 

surface  layers  where  the  nerve-cells  are  concerned  with  special 
organs;  for  example,  there  are  such  centers  for  muscular 
movements  concerned  in  speech,  for  hearing,  taste  and  smell, 
sight,  touch,  and  voluntary  movements  of  various  organs. 
This  discovery  of  localized  centers  in  the  cerebrum  is  already 
valuable  to  surgeons.  For  instance,  if  an  injury  to  a  brain 
interferes  with  any  of  the  functions  named  above,  the  sur- 
geon knows  approximately  where  to  look  for  broken  blood- 
vessels and  other  injuries. 

The  discovery  of  the  centers  controlling  certain  func- 
tions has  no  bearing  upon  the  once-popular  pseudo-science 
called  phrenology.  That  pretended  to  locate  certain  general 
mental  powers  by  the  contour  of  the  skull.  It  was  utterly 
unscientific  because  (1)  there  is  no  evidence  of  such  localiza- 
tion as  phrenology  claimed,  and  (2)  there  is  much  evidence 
against  the  idea  that  the  outer  surface  of  the  skull  indicates 
the  degree  of  development  of  the  brain  beneath. 

ORGANS  OF  SPECIAL  SENSES 

442.  Structure  of  Eye.  —  Use  a  hand-mirror  and  examine 
your  own  eyes  after  reading  the  next  two  paragraphs.  Refer 
frequently  to  Fig.  164. 

The  center  of  the  outer  coat  of  the  eyeball  in  front  is  the 
transparent  cornea,  through  which  one  can  look  into  the 
interior  of  the  eye.  All  the  remainder  of  the  outer  coat  of  the 
eyeball  is  hard,  white,  and  opaque  (the  sclerotic  coat).  One 
might  imitate  the  external  appearance  of  an  eyeball  by  paint- 
ing a  glass  ball  white,  excepting  a  circular  spot  to  represent 
the  transparent  cornea. 

Inside  the  cornea,  and  of  nearly  the  same  size,  is  the  black, 
gray,  or  blue  part  of  the  eye  with  a  hole  in  the  center.  The 
color  pigment  is  in  a  sort  of  thin  membrane  (called  iris), 
and  the  hole  in  the  membrane  is  the  pupil.  Just  back  of  the 
pupil  is  the  lens,  which  is  bi-convex,  transparent,  and  elastic, 


516  APPLIED  BIOLOGY 

so  that  by  pressure  its  shape  can  be  changed.     Back  of  the 
lens  is  the  sensitive  membrane   (retina),  which  is  closely 

attached  to  the  back 
wall  of  the  eyeball, 
and  hence  is  hemi- 
spherical in  shape. 
From  near  the  center 
of  the  retina  the  optic 
y  nerve  extends  to  the 
brain. 

The  space  between 

FIG.  164.     Diagram  of  human  eye.     w,  outer  or  the  lens  and  the  ret- 

sclerotic   wall ;    y,  inner   or  choroid  wall ;    r,  ma    jg    filled    with    a 
retina  (dotted  line)  ;  c,  cornea  ;  t,  iris  ;  I,  lens  .    ,,      ,., 

with     its     suspending    capsule;     v,    vitreous  transparent  jelly-llKe 

humor;   TO,  optic  nerve;    1-2,  a.u  object,  and  substance  (vitrCOUS 

°ntheretina'    (FromHou°h  humor).     The    small 

space  between  the 
lens  and  the  cornea  is  filled  with  a  watery  fluid  (aqueous 
humor).  If  an  eyeball,  obtained  from  a  meat-market,  be 
punctured,  these  humors  escape,  and  the  eyeball  collapses. 
They  are  so  transparent  as  not  to  interfere  with  the  passage 
of  light  from  the  cornea  to  the  retina. 

Between  the  retina  and  the  outer  white  coat  (sclerotic) 
is  a  layer  of  tissue  (choroid)  with  abundant  blood-vessels  and 
black  pigment.  The  pigment  in  the  choroid  and  iris  pre- 
vents light  from  entering  the  eye  except  through  the  pupil 
and  the  lens.  In  the  same  way  black  pigment  in  a  photo- 
graphic camera  prevents  light  from  reaching  the  sensitive 
plate  or  film  except  through  the  lens  and  the  opening  of 
the  diaphragm  which  corresponds  in  use  to  the  iris  and  its 
opening. 

In  both  a  camera  and  an  eye  the  whole  structure  is  essen- 
tially a  light-proof  box  with  a  diaphragm  or  iris  for  regulating 
the  amount  of  light,  and  a  lens  for  focusing  the  rays  of  light 
upon  a  sensitive  plate  or  retina.  The  chief  differences  be- 


HUMAN  STRUCTURE  AND  LIFE- ACTIVITIES       517 

tween  a  camera  and  an  eye  are :  (1)  the  eye  is  living  tissue ; 
(2)  the  sensitive  plate  or  retina  of  the  eye  contains  living 
nerve-cells  connected  to  the  brain  by  nerve-fibers  of  the 
eye-nerve  or  optic  nerve;  and  (3)  the  lens  of  the  eye  is  focused, 
not  as  in  a  camera  by  moving  the  sensitive  plate  nearer  to  or 
farther  from  the  lens,  but  by  muscles  which  change  the  shape 
of  the  eye-lens. 

The  adjustment  of  the  lens  to  suit  different  distances  of 
objects  seen  is  called  accommodation.  It  is  accomplished  as 
follows :  The  lens  when  not  compressed  is  very  -bi-convex, 
as  may  be  seen  in  a  lens  cut  from  an  eye  obtained  at  a  meat- 
market.  When  an  eye  is  resting  in  sleep  or  is  looking  at  far- 
away objects,  the  lens  is  much  flattened  (or  made  less  convex) 
by  the  pull  of  the  elastic  choroid  upon  the  transparent  cap- 
sule which  incloses  the  lens  and  attaches  it  to  the  choroid. 
In  order  to  see  clearly  near-by  objects  the  lens  must  be 
focused  by  being  made  more  convex.  This  is  simply  the 
elastic  return  of  the  lens  toward  its  natural  very  bi-convex 
shape,  and  this  return  is  permitted  by  a  sheet-like  circular 
muscle  which  opposes  the  elastic  pull  of  the  choroid  and 
thereby  eases  the  tension  upon  the  lens. 

It  is  evident  from  the  above  that  the  feeling  of  strain 
when  we  look  at  very  small  objects  is  due  to  the  pull  of 
muscles  against  the  constant  elastic  pull  of  the  choroid 
upon  the  capsule  that  incloses  the  lens. 

(D)  In  order  to  study  the  effect  of  change  of  shape  of  the  lens 
upon  the  focus  of  the  eye,  first  set  up  a  photographic  camera  and 
focus  upon  near  and  distant  objects  by  moving  the  lens.  If  one  had 
lenses  for  different  distances  (very  bi-convex  for  near-by,  and  less 
so  for  far-away)  the  distance  from  the  lens  to  the  sensitive  plate 
might  be  kept  stationary  in  a  camera.  In  the  case  of  an  eye  the 
distance  to  objects  seen  varies,  and  there  is  need  of  many  lenses  of 
different  curvatures ;  or  better  still,  of  one  elastic  lens  whose  shape 
can  be  changed  to  fit  objects  at  any  distance. 

Eyes  that  cannot  see  distant  objects  clearly  are  said  to 
be  "  near-sighted,"  and  one  with  such  eyes  must  hold  print 


518  APPLIED  BIOLOGY 

very  near  in  order  to  bring  it  into  focus.  This  is  due  to 
the  lens  being  abnormally  distant  from  the  retina.  Concave 
glasses  should  be  worn  in  order  to  change  the  direction  of 
rays  of  light  and  cause  them  to  focus  on  the  retina.  Other 
eyes  are  "  far-sighted,"  and  only  by  constant  strain  can  the 
lens  be  kept  convex  enough  to  focus  the  rays  on  the  retina, 
which  is  abnormally  near  the  lens.  Even  when  straining 
to  the  utmost  some  persons  must  hold  books  at  arm's  length 
in  order  to  read.  Convex  glasses  correct  such  difficulty 
and  relieve  the  excessive  strain.  In  old  age  the  lens  loses 
some  of  its  elasticity  and  fails  to  become  convex  enough 
when  trying  to  read,  and  hence  a  book  must  be 
held  at  a  distance,  unless  convex  glasses  are  used.  Astig- 
matism is  a  very  common  congenital  or  inborn  defect  due 
to  irregular  curvature  of  the  cornea  or  lens,  making  it  im- 
possible to  see  equally  well  lines  which  run  in  different 
directions,  as  on  a  clock-face.  For  such  eyes  so-called 
"  cylindrical  "  glasses  should  be  used  constantly  to  avoid 
eye-strain. 

443.  The  Ear.  —  The  human  organ  popularly  known  by 
this  name  is  the  external  ear,  from  which  a  tube  leads  inward  to 
the  tympanic  membrane,  or  ear-drum.  Beyond  this  is  a  cavity 
known  as  the  middle  ear,  and  from  it  the  Eustachian  tube  leads 
to  the  pharynx.  Still  deeper  in  the  head  than  the  middle 
ear  is  the  inner  ear,  a  complicated  membranous  structure 
lying  in  bony  cavities  of  corresponding  shape.  The  lower 
part  (cochlea)  is  shaped  like  the  cavity  in  a  snail's  shell 
(Fig.  165),  and  the  upper  part  has  three  ring-like  canals 
(semi-circular  canals).  Branches  of  the  auditory  nerve  con- 
nect the  sensitive  membranes  of  the  inner  ear  with  the 
brain. 

The  inner  ear  is  filled  with  a  fluid.  Sound  vibrations 
enter  the  outer  tube,  and  throw  the  tympanic  membrane 
into  vibration.  This  moves  a  chain  of  three  bones  which 
extend  across  the  cavity  of  the  middle  ear  and  touch 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       519 


the  membranous  wall  of  the  inner  ear.  Through  this  mem- 
brane vibrations  are  passed  on  to  the  fluid  of  the  inner  ear, 
and  its  vibrations 
stimulate  the  sen- 
sory endings  of  the 
auditory  nerve 
which  are  sensi- 
tive to  sound. 
The  semicircular 
canals  give  us  the 
sense  of  equilib- 
rium, of  which  we 
are  aware  even 
when  we  are  blind- 
folded. 

444.  Smell, 
Taste,  Touch, 
Temperature.  — 
These    senses   are 
connected  with 
special  nerves. 

Those  of  smell  have  endings  in  the  epithelium  which  lines  cer- 
tain upper  cavities  of  the  nose.  The  nerves  of  taste  end  in  the 
little  projections  (papillae)  on  the  tongue.  Those  of  touch  and 
temperature  are  widely  distributed  in  the  skin  of  all  parts  of 
the  body. 

THE  SKIN  AND  ITS  WORK 

445.  Human  Skin.  —  Microscopic  preparations  show  that 
the  surface  of  the  skin  is  made  up  of  closely  set  cells,  while  the 
lower  side  next  to  the  muscles  and  bones  is  made  up  of  con- 
nective tissue  (Fig.  166).     The  cellular  layer  is  the  epidermis, 
and  the  connective  tissue  is  the  dermis. 

(D)   A  piece  of  leather  tanned  without  the  hair  will  give  a  good 
view  of  the  intricately  interlaced  fibers  of  the  dermis,  the  epidermis 


FIG.  165.  Diagram  of  human  ear.  a,  canal  from 
external  ear ;  t,  tympanum ;  m,  middle  ear 
with  small  bones  extending  from  tympanum  to 
inner  ear  ;  s,  one  of  the  semicircular  canals  of 
hmer  ear ;  c,  cochlea  of  inner  ear ;  n,  auditory 
nerve  ;  6,  bone  surrounding  middle  and  inner 
ears ;  e,  Eustachian  tube,  and  p,  its  opening  to 
pharynx. 


520 


APPLIED  BIOLOGY 


having  been  removed  in  the  process  of  tanning.     Soften  such  a  piece 
of  leather  by  soaking  in  hot  water,  and  then  examine  by  tearing  it 

apart. 

(L)   The  dermis  is  tied  down  to  deeper  tissues  by  connective-tissue 

fibers,  many  of  them  elastic.    Pull  up  the  skin  on  back  of  your  hand, 

and  note  how  quickly  it 
returns  to  place  when 
released. 

With  a  hand-lens  ex- 
amine the  skin  on  arm  or 
hand.  Note  the  delicate 
ridges  and  grooves,  es- 
pecially on  the  finger- 
tips. Press  the  finger- 
tips on  an  ink-pad  and 
then  make  prints  on 
paper.  No  two  persons 
have  the  same  finger- 
prints; and  so  this  has 
proved  to  be  a  valuable 
means  of  identification, 
especially  in  records  of 
criminals. 


The  surface  of  the 
skin  looks  scaly  as 


FIG.  166.  Section  of  human  skin.  Se,  outer 
hard  layer  of  dry  cells  ;  SM,  layer  of  living 
cells  ;  Co,  the  dermis  (connective  tissue).  Se 
and  SM  together  constitute  the  epidermis. 
G,  blood-vessels  ;  N,  nerves  ;  NP,  nerve-end-  seen  Under  a  strong 

*SK  -F'  f/£;  ^V7at^anvds  and  ducts;  lens.    These  scales  are 

H,  hair.     (From  Wiedersheim.}  .. 

dried  cells.  They  swell 

in  solutions  of  caustic-potash  or  washing-soda,  and  pieces 
scraped  from  a  calloused  spot  on  the  palm  may  be  so  treated 
in  preparing  for  microscopic  examination.  This  swelling 
when  wet  explains  why  the  hands  get  so  soft  when  kept  for 
a  time  in  water,  especially  if  soapy. 

A  microscopic  section  shows  that  the  scaly  cells  at  the 
surface  are  many  rows  deep,  and  that  deeper  down  in  the 
epidermis  are  several  rows  of  rounded  or  cubical  cells,  which 
are  usually  brightly  stained  in  preparations.  These  are  living 
cells,  while  the  hard  scaly  ones  at  the  surface  are  dead.  The 


'HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       521 

scaly  cells  together  constitute  the  horny  layer,  and  the  lower 
cells  of  the  epidermis  form  the  Malpighian  layer,  or  living 
layer.  The  horny  dead  cells  at  the  surface  are  continually 
wearing  off,  and  the  living  layer  cells  are  by  division  forming 
new  cells,  which  are  pushed  toward  the  surface. 

In  warts,  corns,  and  callouses  the  horny  layer  becomes 
greatly  thickened.  The  cause  is  growth  and  division  of  the 
lower  living  cells  faster  than  the  hard  cells  at  the  surface 
wear  off.  Friction,  pressure,  and  introduction  of  some 
foreign  substance  through  a  cut  or  scratch  are  some  of  the 
well-known  causes  of  these  thickenings  of  epidermis. 

In  the  ordinary  healing  of  a  cut  or  burn,  the  cells  of  the 
living  layer  quickly  form  new  cells  to  fill  the  gap  beneath  the 
blood-clot  or  "  scab  "  which  forms  on  the  surface.  In  ex- 
tensive burns  it  is  sometimes  necessary  to  resort  to  skin- 
grafting.  This  means  taking  healthy  pieces  of  skin  from 
other  persons  and  applying  them  where  the  epidermis  has 
been  completely  destroyed.  The  cells  of  these  healthy  bits 
of  grafted  skin  soon  become  firmly  attached  and  by  repeated 
division  grow  over  the  injured  surface. 

Nails  and  hairs  are  specialized  masses  of  horny  cells.  The 
so-called  "  roots  "  of  nails  and  hairs  are  deep-lying  masses  of 
living  cells  which  grow  and  divide  rapidly.  The  pit  in  which 
each  hair  is  attached  is  called  a  hair-follicle,  and  in  it  is  a 
conical  elevation  (papilla)  from  which  the  hair  grows  as  cells 
are  formed  and  pushed  outward.  Oil  glands  (sebaceous 
glands)  open  into  the  hair-follicles.  Cutting  or  shaving  hair, 
contrary  to  popular  belief,  does  not  increase  the  number  of 
hairs,  for  the  follicles  are  formed  in  embryonic  life. 

(L)  Examine  a  hair  with  microscope,  and  note  that  it  is  composed 
of  overlapping  scales  (cells).  The  center  is  also  filled  with  cells. 
The  idea  of  barbers  that  hairs  are  hollow  and  require  singeing  to 
prevent  the  escape  of  oil  is  absurd  —  but  profitable  to  the  barber. 

Sweat-glands  are  abundant  over  the  whole  human  skin. 
They  are  most  numerous  on  palms  and  soles,  which  also  have 


522  APPLIED  BIOLOGY 

no  hairs.  Each  sweat-gland  is  a  tube  extending  from  the  so- 
called  "  pore  "  down  into  the  dermis,  where  it  is  much  coiled 
and  surrounded  by  blood-capillaries. 

446.  Functions  of  the   Skin.  —  Next   to   protection,   the 
most  important  function  of  the  human  skin  is  heat  regulation. 

While  the  soft  skin  of  frogs  and  other  lower  animals  is  im- 
portant for  respiration,  the  dry  hard  skin  of  mammals  and 
man  is  of  little  use  in  this  way.  It  has  been  shown  by  putting 
a  man's  body  in  a  rubber  bag  which  was  tightly  fitted  around 
the  neck  that  the  lungs  give  off  nearly  two  hundred  times  more 
carbon  dioxide  than  the  skin  does.  Hence,  skin  respiration 
is  of  no  practical  importance  to  us. 

Absorption  by  the  skin  is  of  little  importance.  Oily 
materials  are  often  rubbed  into  the  skin,  and  small  quantities 
appear  to  be  absorbed.  Probably  the  massage  effect  is  most 
important.  It  is  known  that  some  drugs  can  be  absorbed 
when  thus  rubbed  into  the  skin. 

Secretion  of  Sweat. — Under  ordinary  temperature  conditions 
the  amount  of  secretion  does  not  attract  attention,  because  it 
is  evaporated  as  rapidly  as  formed.  Large  quantities  of  water 
taken  into  the  stomach  are  absorbed,  raise  the  blood-pressure, 
and  increase  perspiration.  Certain  drugs  bring  about  dila- 
tion of  blood-vessels  and  cause  profuse  sweating;  others 
act  in  an  opposite  manner. 

Normal  sweat  is  nearly  99  per  cent  water.  The  one  per 
cent  of  dissolved  substances  are  chiefly  mineral  excretions 
similar  to  those  in  the  urine.  If  the  skin  were  varnished,  the 
same  excretion  would  be  easily  eliminated  by  the  kidneys. 
Sweat,  then,  is  not  necessary  as  an  excretion,  but  as  an  aid 
to  heat  regulation  (see  next  section). 

447.  Skin  as  Heat-Regulator.  —  The  fact  that  the  notice- 
able activities  of  the  sweat-glands  are  usually  associated  with 
great  internal  heat  suggests  that  the  chief  function  of  these 
glands  is  not  so  much  to  get  rid  of  sweat  (which  the  kidneys 
could  manage)  as  to  discharge  heat. 


HUMAN  STRUCTURE  AND  LIFE-ACTIVITIES       523 

The  muscular  system  is  the  chief  source  of  increased  heat 
production,  while  the  other  organs  probably  generate  the  heat 
which  is  more  or  less  constant,  as  during  sleep  or  complete 
rest.  When  these  organs  are  unable  to  supply  the  necessary 
heat,  shivering,  which  is  involuntary  activity  of  muscles, 
may  begin  and  thus  increase  the  internal  supply  of  heat. 

Loss  of  heat  is  partly  by  respiration.  This  is  very  impor- 
tant for  dogs,  which  "  pant  "  when  overheated  because  their 
skin  perspires  little  except  on  the  pads  of  the  feet.  They 
also  lose  heat  rapidly  from  the  evaporation  that  occurs  on 
the  surface  of  the  protruded  tongue.  Chickens  are  often 
seen  breathing  rapidly  with  the  mouth  open ;  and  thus  birds 
lose  heat  from  their  lungs  and  air-sacs.  In  man,  however, 
the  skin  is  the  one  great  heat-regulator. 

The  loss  of  heat  from  the  human  skin  is  controlled  by 
nerves,  some  of  which  regulate  (§  414)  the  flow  of  blood  to 
the  skin  and  sweat-glands,  and  some  stimulate  these  glands 
into  activity.  Rapid  exercise  causes  the  sweat-glands  to 
become  active.  The  effect  of  the  sweat  is  illustrated  by  the 
familiar  cooling  of  bottles  of  water  wrapped  with  wet  towels 
exposed  to  warm  air ;  of  water  allowed  to  evaporate  from  the 
hands  or  face  on  a  summer's  day;  or  of  the  porous  water- 
jugs  and  canvas  bags  which  the  inhabitants  of  some  hot 
countries  use  for  their  drinking  water  because  the  small 
amount  of  oozing  water  is  evaporated  and  cools  the  water 
in  the  jug.  We  also  know  that  following  a  bath  on  a  hot 
day  the  body  cools  rapidly,  and  the  explanation  is  that  the 
heat  of  the  skin  was  used  to  evaporate  the  water.  Now,  the 
sweat-glands  are  simply  mechanisms  for  covering  the  skin 
with  water  ready  for  evaporation  at  all  times  when  the  skin 
is  warmed  by  blood  circulating  rapidly. 

In  addition  to  the  heat  lost  by  evaporation  of  water  on 
the  surface,  there  is  much  loss  by  radiation,  especially  in  cold 
weather  when  the  skin  gets  warm  after  exercise  which  is  not 
often  active  enough  to  cause  moistening  the  skin  with  sweat. 


524  APPLIED  BIOLOGY 

Certainly  at  such  times  the  warmer  skin  must  radiate  more 
heat  to  the  air,  just  as  a  hot-water  pipe  does  when  the  tem- 
perature of  the  circulating  water  increases. 

The  importance  of  the  evaporation  of  sweat  as  a  method 
of  cooling  depends  upon  the  temperature  and  humidity  of  the 
air,  which  determine  the  amount  of  water  the  air  can  absorb. 
Dry  cold  air  can  take  up  little  watery  vapor,  while  dry  warm 
air  may  contain  much  more  water  than  the  cold  air.  Hence 
in  hot  weather  loss  of  heat  by  the  evaporation  of  sweat  be- 
comes more  important.  Obviously,  hot  dry  winds  would  favor 
evaporation;  while  hot  moisture-laden  air  prevents  rapid 
heat  loss  both  by  radiation  and  by  evaporation.  This  is  the 
usual  condition  on  oppressively  hot  days  when  heat  pros- 
trations and  sunstrokes  are  common.  The  moist  hot  air 
prevents  proper  loss  of  heat  from  the  skin,  and  the  internal 
temperature  rises  too  far  above  99°  F.,  which  is  best  for 
human  protoplasm.  Obviously,  reduced  heat  production 
will  help  avoid  excessive  internal  heat ;  and  this  means  keep- 
ing as  quiet  as  possible  and  eating  sparingly,  thus  reducing 
the  activity  of  muscles  and  the  digestive  organs.  (Why  do 
people  in  tropical  climates  with  hot  sun  and  moisture-laden 
winds  suffer  from  the  heat  more  than  do  the  inhabitants  of 
equally  hot  but  dry  regions  ?) 

Fever  is  due  to  increased  production  of  heat  (caused  by 
toxins  of  disease),  and  is  usually  accompanied  with  dry  skin 
and  inactive  sweat-glands;  but  occasionally  even  great 
perspiration  does  not  discharge  the  heat  fast  enough.  The 
so-called  "  wasting  "  indicated  by  loss  of  weight  during  pro- 
longed fever  is  due  to  the  rapid  oxidation  of  tissues  when 
little  or  no  food  is  available.  Drugs  act  upon  fevers  by  re- 
ducing oxidation  in  tissues  or  by  promoting  perspiration  and 
consequent  heat  loss  from  the  skin. 

The  effect  of  cold  baths  upon  heat  regulation  by  the  skin 
is  discussed  in  §  458. 


CHAPTER  XVIII 

PRINCIPLES  OF  BIOLOGY  APPLIED  TO   HEALTHFUL 

LIVING 

I.    PERSONAL  HYGIENE 

448.  Hygiene.  —  In  this  chapter  it  will  be  shown  that 
very  many  of  the  facts  and  principles  of  biology  are  of  great 
value  when  applied  to  making  the  human  body  freer  from 
disease  and  a  more  efficient  mechanism  for  carrying  on  a  use- 
ful and  happy  life. 

That  department  of  biological  science  which  deals  with  the 
established  principles  of  human  health  is  commonly  known 
as  hygiene.  It  is  simply  a  phase  of  applied  biology.  When- 
ever hygiene  deals  with  the  health  of  individuals,  how  to  take 
care  of  oneself,  as  in  rules  for  eating,  breathing,  sleeping,  etc., 
it  is  called  personal  hygiene.  This  is  chiefly  the  principles 
of  biology,  particularly  of  physiology,  applied  with  a  good 
proportion  of  common  sense.  By  this  we  mean,  for  example, 
that  one  who  has  studied  the  structure  and  working  of  the 
lungs  will  need  only  common  sense  to  enable 'him  to  see  that 
tight  clothing  will  interfere  with  the  natural  movements,  and 
then  the  hygienic  rule  "  do  not  wear  tight  clothing  "  is  seen 
to  be  a  scientific  application  of  the  biological  principles  re- 
lating to  human  breathing.  The  same  is  true  with  regard 
to  every  organ;  and  we  do  well  to  examine  every  proposed 
new  rule  of  hygiene  from  the  standpoint  of  applied  biology. 
Thousands  of  foolish  rules  of  hygiene  have  been  published, 
but  it  is  usually  possible  to  select  the  good  ones  because  they 
are  obviously  based  upon  the  principles  of  biology.  For 
example,  if  some  one  advised  prolonged  mastication  of  fat 

525 


526  APPLIED  BIOLOGY 

meat,  we  should  reject  this  absurd  rule  at  once,  because  biology 
teaches  that  such  food  is  not  digested  by  saliva.  We  should 
keep  a  sharp  lookout  for  such  applications  of  biology  to 
the  unscientific  rules  of  personal  hygiene  which  so  many 
people  accept  without  question. 

Public  hygiene  or  sanitation  means  principles  of  biology 
applied  to  increasing  the  health  of  a  community  of  people. 
Problems  relating  to  clean  streets,  pure  food-supply,  in- 
fectious diseases,  sewerage,  water-supply,  and  others  under 
health  officers,  belong  to  public  hygiene. 

RESPIRATORY  ORGANS 

449.  Habits  of  Breathing.  —  The  effect  of  tight  clothing 
is  referred  to  in  the  preceding  section.  All  authorities  in  medi- 
cine and  hygiene  unite  in  condemning  the  fashion  of  wearing 
any  clothing  which  interferes  with  breathing  movements. 

Exercise  in  deep  breathing  is  important,  for  it  trains  the 
respiratory  muscles  so  that  fuller  expansion  of  the  lungs 
occurs  regularly.  This  means  that  fresh  air  goes  deeper  into 
the  air-tubes  (see  §§  428,  429).  Consult  your  physical-train- 
ing teacher  as. to  the  advantage  of  training  in  breathing  for 
athletics. 

Breathing  through  the  nose  is  the  natural  way,  because  the 
air  is  properly  warmed  and  much  dust  is  stopped  in  the 
nasal  passages.  Mouth-breathing  is  abnormal,  but  very 
common.  Children  who  breathe  habitually  through  the 
mouth  probably  have  enlarged  adenoids  in  the  upper  part  of 
the  pharynx  back  of  the  soft  palate.  If  not  removed  at  once 
by  a  competent  surgeon,  they  may  seriously  interfere  with 
the  health  or  cause  deafness,  or  even  deformation  of  the  upper 
They  commonly  disappear  after  childhood,  but  then 

3  damage  has  been  done.  Hence  any  special  difficulty 
with  natural  breathing  through  the  nose  should  be  referred 
at  once  to  a  doctor  for  advice. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      527 

450.  Ventilation.  —  Frequent  renewal  of  the  air  of  build- 
ings is  required ;  because  the  breathing  of  human  beings  and 
the  combustion  in  stoves,  lamps,  etc.,  use  oxygen  and  add 
carbon  dioxide ;  and  because  bacteria  are  carried  in  the  dust 
of  the  air.  All  systems  of  ventilation  should  provide  for  the 
exit  of  the  lighter  warm  air  near  the  ceiling  and  the  entrance 
of  fresh  air  (preferably  warmed  previously  by  heaters)  near 
the  floor.  At  night  when  the  body  is  properly  protected  in 
bed  the  supply  of  fresh  air  may  safely  be  very  cold.  The  im- 
portant rule  is  to  get  as  much  fresh  air  as  possible  without 
chilling  any  part  of  the  body  by  drafts.  That  is  far  more 
harmful  than  poor  ventilation,  for  "  colds  "  are  often  pro- 
duced (§  453).  Good  ventilation  does  not  require  that  a  high 
wind  constantly  blow  through  a  house. 

For  accounts  of  the  best  methods  of  ventilation,  see  books 
of  hygiene  and  household  science.  The  subject  is  so  extensive 
that  only  the  great  principles  can  be  suggested  here. 
>  451.  Avoiding  Dust.  —  Owing  to  the  fact  that  dust  often 
carftes-ebngerous  bacteria  and  that  there  is  a  harmful  effect 
of  'accumulated  dust  in  the  lungs,  dust  in  the  air  should  be 
eliminated  as  far  as  possible  from  homes,  factories,  and  public 
buildings.  Old-fashioned  sweeping  and  dusting  are  dan- 
gerous unless  windows  are  opened  and  the  wind  allowed  to 
blow  out  the  dust  and  bacteria.  Carpet-sweepers  and  es- 
pecially vacuum-cleaners  are  better.  Damp  cloths  should 
be  used  for  wiping  dust  from  furniture ;  brushes  and  feather- 
dusters  are  relics  of  the  dark  ages  and  should  never  be  used. 
If  brooms  must  be  used  indoors,  damp  sawdust  will  help  keep 
dust  from  rising  into  the  air.  The  most  sanitary  modern 
dwellings  have  no  carpets  which  are  not  easily  taken  outdoors 
for  cleaning,  while  the  floors  are  painted,  varnished,  or  waxed 
so  as  to  make  washing  easy  and  sweeping  unnecessary. 

452.  Artificial  Breathing.  —  This  means  causing  the  lungs 
to  respire  after  normal  breathing  has  been  stopped  by  im- 
mersion in  water  or  by  gas  asphyxiation.  This  can  be  done 


528  APPLIED  BIOLOGY 

by  placing  the  patient  on  his  back  and  regularly  (fifteen 
times  a  minute)  raising  the  arms  above  his  head. and  then 
gently  lowering  them  to  the  sides,  making  the  chest-walls 
move  as  in  natural  breathing.  For  details  of  this  and  other 
methods,  see  special  chapters  on  "  accidents  "  in  books  on 
hygiene.  The  teacher  should  give  practical  lessons  on  this 
important  topic,  selecting  one  student  as  patient  and  another 
as  operator. 

453.  Colds    in    Respiratory    Organs.  —  Severe    "  colds  " 
which  are  liable  to  lead  to  bronchitis  (inflammation  of  bron- 
chial tubes),  pleurisy  (inflammation  of  pleura  of  lungs),  or 
pneumonia    (congestion  of  blood,  with    certain    poisonous 
bacteria  in  the  lungs)  require  medical  advice.     Chronic  con- 
gestion of  nasal  membranes  leading  to  the  condition  known 
as  catarrh  should  receive  medical  attention,  for  a  slight  opera- 
tion or  special  treatment  may  be  necessary  to  effect  a  cure. 

For  preventing  colds  see  §  458  on  bathing.  One  whose 
skin  has  been  chilled  should  hasten  to  restore  the  normal  cir- 
culation of  the  skin  by  exercise,  friction,  hot  drinks,  hot  bath 
(with  great  precaution  against  more  chilling),  and  in  extreme 
cases  by  certain  drugs  which  physicians  advise.  It  is  dan- 
gerous to  neglect  a  "  cold,"  especially  in  its  incipient  stages. 

DIGESTIVE   ORGANS 

454.  Teeth.  —  The  proper  care  of  the  teeth  is  commonly 
emphasized  in.  the  books  on  "  physiology  "  written  for  ele- 
mentary schools,  and  need  not  be  repeated  here.     The  fact 
that  bacteria  are  largely  responsible  for  dental  decay  suggests 
the  daily  use  of  antiseptic  tooth-powders  and  mouth-washes. 

455.  Hygiene   of  Eating.  —  The    practicable    hygiene   of 
digestion  for  most  people  is  that  which  suggests  the  time 
for  eating,  and  the  amount  and  kind  of  food. 

The  question  of  the  time  of  meals  depends  upon  other  physio- 
logical demands.  A  light  breakfast  and  a  light  lunch  are  best 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      529 

for  busy  people,  whose  muscles  or  nerves  are  hard-worked. 
Then,  after  the  day's  work  and  a  brief  rest  is  the  best  time  for 
dinner,  the  chief  meal  of  the  day.  The  reason  for  this  is  that 
intellectual  activity  leads  to  a  marked  increase  in  the  amount 
of  blood  in  the  vessels  of  the  brain,  and  physical  work  affects 
muscles  similarly.  The  result  must  be  a  withdrawal  of 
blood  from  the  vessels  of  the  digestive  organs  and  a  conse- 
quent retardation  of  the  digestive  process. 

The  above  plan  suits  most  people  ;A.4)ut  there  are  numerous 
individual  exceptions,  and  each  one  must  experiment  with 
himself  if  he  would  find  the  most  v  satisfactory  time  for  meals. 
Work  and  habits  of  human  beings  vary  so  greatly  that  there 
can  be  no  universal  law  of  eating.  Certain  rules  for  general 
application  have  been  established  by  the  experience  of  thou- 
sands of  people,  and  probably  most  important  for  remember- 
ing are  the  following:  (1)  There  should  be  regularity  in 
meals.  (2)  Physical  and  mental  fatigue  interfere  with  di- 
gestion. (3)  Overeating  acts  likewise. 

The  selection  of  diet  and  its  amount  is  similarly  variable. 
Hard  physical  exertion  and  exposure  to  cold  demand  abun- 
dant food  for  energy  (§§  420,  421),  and  the  question  of  easy 
digestibility  is  of  minor  importance.  On  the  other  hand,  per- 
sons of  sedentary  habits  should  avoid  unnecessary  amounts  of 
all  kinds  of  foods ;  and  also  observe  well  their  own  pecul- 
iarities as  to  digestibility  of  carbohydrates  and  fats. 

Eating  anything  between  meals  is,  as  a  rule,  inadvisable; 
but  here  again  there  are  individual  exceptions,  and  knowledge 
of  the  possible  harm  to  digestion  will  lead  to  caution. 

Some  water  (not  iced)  should  be  taken  at  every  meal,  for 
it  is  needed  in  liquefying  the  contents  of  the  stomach  in  prep- 
aration for  escape  into  the  intestine  (§  400).  But  water 
should  not  be  taken  at  the  same  moment  with  solid  food,  for 
it  "  washes  down  "  the  food  and  thus  prevents  mastication. 
The  old  idea  that  water  dilutes  the  gastric  juice  does  not 
now  seem  to  be  very  significant,  for  it  has  been  discovered 
SM 


530  APPLIED  BIOLOGY 

recently  that  water  soon  passes  from  the  stomach  into  the 
intestine  and  gastric  juice  is  secreted  rapidly. 

Overeating  is  the  chief  point  on  which  many  people  need 
physiological  advice.  Scientific  studies  have  often  shown 
that  men  need  no  more  food  than  stated  in  §  419,  and  yet 
a  large  number  of  people  take  more  daily.  Especially  is  it 
true  that  we  use  too  much  protein  (§  423),  thus  unnecessarily 
overworking  all  organs  without  any  gain.  The  other  foods 
are  also  used  to  excess  by  many  whose  daily  activities  do  not 
require  so  much  stored  energy;  and  the  common  result  is 
fat-storage,  which  often  becomes  so  excessive  as  to  be  un- 
comfortable or  even  a  danger  to  the  heart  and  other  organs. 
In  the  majority  of  cases  excessive  fat  storage  is  due  to  the 
overeating  of  foods  containing  carbon,  hydrogen,  and  oxygen 
(sugar,  starch,  butter,  and  fat  meat).  The  over-fat  condition, 
once  established,  is  difficult  to  change;  and  hence  young 
people  should  guard  against  an  excessive  increase  in  stored 
fat  due  to  intemperate  eating. 

An  excess  of  meat  diet  is  more  harmful  than  an  excess  of 
other  foods,  for  the  reason  that  most  proteins  eaten  are  oxi- 
dized and  excreted  within  a  day  (§  422).  The  nitrogen  excre- 
tions thus  formed  from  proteins  may  play  an  important  part 
in  the  development  of  gouty  and  rheumatic  conditions. 

The  value  of  mastication  has  been  the  subject  of  much 
discussion,  and  is  still  uncertain ;  for  there  are  some  people 
who  masticate  little  and  have  perfectly  healthy  digestion, 
and  there  are  others  who  masticate  extensively  and  claim  to 
have  thereby  cured  indigestion.  The  truth  is  that  it  is 
largely  a  question  of  the  kind  and  amount  of  food  and  the 
habits  of  the  individual.  Certainly  one  who  eats  an  excessive 
amount  of  starchy  food,  or  who  has  starch-indigestion  in  the 
intestine,  will  do  well  to  masticate  starchy  food  and  allow 
the  saliva  to  exert  its  digestive  influences  as  long  as  possible, 
for  thereby  he  may  cure  a  form  of  indigestion.  But  this  does 
not  prove  a  rule  for  all  people  or  for  all  articles  of  diet ;  and 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      531 

with  no  more  than  the  necessary  amount  of  properly  cooked 
food  the  average  individual  can  safely  follow  his  natural 
instincts  as  to  the  amount  of  mastication.  There  are  those 
who  point  to  the  rumination  of  cows  and  sheep  for  evidence 
that  naturally  animals  masticate  food  for  a  long  time;  but 
this  gives  no  rule  for  human  guidance.  In  the  first  place, 
a  cow's  natural  food  is  uncooked  and  otherwise  unprepared ; 
second,  her  digestive  organs  are  quite  unlike  the  human; 
and,  third,  dogs  more  closely  resemble  man  in  structure  and 
in  foods  and  they  never  masticate.  Obviously,  it  cannot  be 
concluded  that  man  should  chew  his  food  long  because  cows 
and  sheep  do.  The  only  scientific  conclusion  must  be  based 
upon  individual  human  experience;  and  this  indicates  that 
some  people  with  weak  digestion  of  starch  need  to  give 
special  attention  to  mastication,  but  that  most  people  may 
safely  forget  their  jaws  while  eating  (i.e.,  masticate  instinc- 
tively) provided  that  they  do  not  eat  too  rapidly  or  in  excess. 
However,  it  is  well  for  each  person  to  experiment  upon  him- 
self, and  thus  determine  how  far  special  attention  to  masti- 
cation is  important  for  himself. 

One  point  in  favor  of  thorough  mastication  deserves  men- 
tion, namely,  that  it  tends  to  prevent  overeating.  When  food 
is  rapidly  swallowed,  there  may  be  an  excess  taken  before  the 
gastric  nerves  give  us  warning  of  too  much  food.  Prolonged 
mastication  tends  to  avoid  this  result,  possibly  because  the 
sugar  obtained  from  the  salivary  digestion  acts  upon  the 
gastric  nerves  just  as  sweets  before  a  meal  "  take  away  the 
appetite  "  or  reduce  it.  However,  it  is  well  never  to  eat  until 
hunger  is  completely  satisfied,  and  this  rule  would  avoid  most 
overeating. 

Stimulants.  —  Should  digestive  stimulants  be  avoided,  is 
a  much-discussed  question,  usually  with  regard  to  alcoholic 
drinks.  It  also  should  apply  to  spices,  condiments,  coffee, 
tea,  cocoa,  carbonated  water,  and  even  hot  food ;  for  these 
all  have  some  stimulating  effect  upon  the  digestive  organs. 


532  APPLIED  BIOLOGY 

It  is  argued  that  stimulants  are  not  natural,  for  animals  do 
not  require  such  things ;  but  it  may  be  answered  that  ani- 
mals do  not  lead  sedentary  lives,  undergo  intense  nervous 
strain,  and  do  similar  things  which  in  civilized  man  tend  to 
interfere  with  proper  digestion.  Certainly  we  can  often  gain 
by  the  temperate  use  of  some  stimulants;  but  harm  will 
always  come  from  overstimulation,  which  is  most  likely  to 
happen  in  the  use  of  tea  and  coffee  (§  480)  and  alcohol  (§  468). 
Probably  the  most  useful  and  safest  of  all  stimulants  are 
hot  foods  and  hot  drinks,  such  as  hot  bouillon,  hot  milk,  or 
even  hot  water. 

456.  Psychology  of  Digestion.  —  We  have  already  defined 
psychology  as  the  science  of  the  mind ;  and  here  it  is  impor- 
tant to  note  that  the  mind  greatly  influences  digestion.  It  is 
well  known  that  the  thought,  sight,  or  smell  of  savory  food 
causes  the  "  mouth  to  water,"  that  is,  stimulates  the  salivary 
glands;  and  there  is  a  similar  effect  on  the  gastric  glands. 
We  also  know  how  certain  disagreeable  mental  states  may 
cause  loss  of  appetite  and  even  nausea.  In  fact,  the  gastric 
glands  may  fail  to  secrete,  and  indigestion  and  other  dis- 
turbances may  be  caused  by  the  direct  influence  of  the 
mind. 

Such  well-known  facts  suggest  the  importance  of  pleasant 
surroundings  while  one  is  eating.  Here  is  the  secret  of  the 
good  digestive  influence  of  music,  jolly  company,  and  other 
things  which  make  for  a  happy  state  of  mind  during  the  meal 
hour.  That  is  a  time  when  every  one  should  drop  his  cares 
and  troubles. 

On  the  other  hand,  there  is  a  certain  real  danger  in  pleas- 
ant accompaniments  of  meals.  It  is  simply  that  fine  fare 
and  surroundings  which  are  agreeable  to  all  the  senses  tend  to 
feasting  or  overeating,  a  result  often  capable  of  unhealthful 
consequences.  The  only  safety  is  in  learning  self-control  so 
that  one's  stimulated  appetite  may  not  lead  on  and  on  to 
gluttony. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      533 

SKIN 

457.  Skin  Cleanliness.  —  The  scientific  reason  for  clean- 
ing the  skin,  especially  by  the  use  of  soap  in  warm  water,  is 
that  it  removes  dirt,  which  is  objectionable  primarily  be- 
cause it  is  unsesthetic  and  therefore  disagreeable  to  refined 
people,  and  secondarily,  because  dirt  contains  micro-organisms 
which  may  produce  disease.  In  fact,  dirty  hands  have  often 
caused  typhoid  and  other  dangerous  disease^  by  leaving 
bacteria  on  dishes  and  on  foods.  Formerly  it  was  supposed 
that  a  third  reason  for  cleansing  the  skin  with  soap  was  to  re- 
move substances  which  "  clog  the  pores  of  the  skin  and  keep 
in  excretions";  but,  as  we  have  seen  in  §446,  the  skin  has 
little  to  do  with  excretions  except  as  water  is  incidentally 
eliminated  while  the  skin  is  reducing  the  internal  heat,  hence 
"  keeping  the  pores  open  "  is  a  weak  argument  for  cleaning 
the  skin.  The  fact  is  that  the  pores  "  open  "  quickly,  even 
on  the  dirtiest  skin,  when  exercise  develops  excess  of  heat. 
However,  we  do  not  need  the  unscientific  theory  of  "  keeping 
pores  open";  for  the  aesthetic  and  bacteriological  reasons 
named  above  are  sufficient  to  convince  any  civilized  person, 
and  especially  any  one  who  sees  the  force  of  the  rule,  "  use 
soap  because  refined  people  do  so  and  the  barbarians  do 
not."  This  is  more  sensible  than  trying  to  show  that  soap 
is  necessary  for  hygienic  reasons,  for  there  are  healthy 
people  who  never  or  rarely  use  soap.  Such  health  is  not 
surprising,  for  soap  has  very  much  to  do  with  aesthetics  and 
little  with  health,  except  in  possible  bacterial  infections. 

The  abuse  of  soap  by  many  refined  people  deserves  atten- 
tion. Exposed  parts  of  the  body  must  be  washed  very  fre- 
quently with  soap,  and  preferably  with  warm  water ;  but  a 
complete  bath  with  soap  and  warm  water  is  taken  too  fre- 
quently by  many  persons.  Soap  removes  the  oily  secretions 
and  the  warm  water  dilates  the  blood-vessels  of  the  skin, 
increasing  liability  to  colds.  A  warm  soapy  tub-bath  once 


534  APPLIED  BIOLOGY 

a  week  should  be  followed  by  cold  water,  which  causes 
contraction  of  the  blood-vessels  and  a  subsequent  reaction 
(§  414). 

As  far  as  is  consistent  with  cleanliness,  soap  with  cold  water, 
and  especially  cold  water  without  soap  should  be  used  daily 
on  all  parts  of  the  body  not  exposed.  The  special  reason 
for  cold  instead  of  warm  water  is  given  in  the  next  para- 
graph. 

458.  Bathing  as  a  Skin  Tonic.  —  Above  we  have  considered 
bathing  for  cleanliness  only ;  but  here  we  are  interested  in 
bathing  for  health.  While  only  in  removing  bacteria  does 
skin  cleanliness  appear  to  be  necessary  for  health,  we  have 
fortunately  a  very  strong  argument  for  daily  bathing  as  a 
means  of  leading  to  healthy  action  of  the  skin  blood-vessels, 
and  indirectly  of  the  whole  body.  For  this  purpose  water 
should  be  much  colder  than  the  temperature  of  the  body, 
and  is  best  applied  as  sponge-bath,  shower-bath,  or  plunge- 
bath  (as  in  sea-bathing).  Contact  with  the  cold  water  first 
causes  a  reflex  action  leading  to  a  reduced  caliber  of  the  skin 
arteries,  and  the  skin  quickly  becomes  pallid.  A  reaction 
follows  brief  exposure  to  the  cold  water,  and  the  blood-vessels 
expand,  the  skin  glows,  and  the  bather  feels  stimulated.  No 
such  good  effect  comes  from  too  long  exposure,  or  when  the 
water  is  too  cold  for  some  people. 

The  explanation  of  the  value  of  this  cold  bathing  is  that 
it  gives  the  skin  practice  in  readjusting  its  blood-supply  when 
exposed  to  a  low  temperature.  Many  persons,  especially 
those  of  sedentary  habits,  have  skins  which  are  not  accustomed 
to  react  quickly  to  changes  of  temperature;  and  hence  if 
chilled  their  skin  arteries  remain  contracted,  blood  which 
ought  to  circulate  in  the  skin  is  congested  in  some  internal 
organ,  where  inflammation  develops,  and  a  "  cold  "  follows. 
Frequent  practice  in  readjusting  the  blood-vessels,  as  given 
by  cold  bathing,  will  make  the  skin  more  likely  to  react  and 
continue  to  receive  its  fair  share  of  blood  whenever  exposed 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      535 

to  low  temperature,  thus  tending  to  avoid  the  dangerous 
internal  congestions  known  as  "  colds."  It  should  be  noted 
that  "  colds  "  are  not  confined  to  the  lungs  and  respiratory 
passages,  for  one  may  have  an  unrecognized  "  cold  "  from 
congestion  of  blood  in  other  organs,  especially  in  stomach, 
intestine,  and  kidneys. 

If  in  spite  of  systematic  cold  bathing  one's  skin  some- 
times gets  so  chilled  that  it  does  not  soon  react,  the  normal 
circulation  should  be  restored  by  ways  suggested  in  §  453. 
To  allow  the  skin  to  remain  chilled  for  hours  is  dangerous. 

Cold  baths  are  best  taken  before  breakfast,  never  within 
two  hours  after  a  meal,  because  the  rush  of  blood  to  the  skin 
interferes  with  the  proper  supply  in  the  digestive  organs. 

NERVOUS  SYSTEM 

459.  Overwork.  —  Unhealthy  conditions  of  the  nervous 
system  are  frequently  the  result  of  overwork,  both  mental  and 
physical.  Hence  it  is  important  that  brain-workers  should 
consider  the  hygiene  of  the  nervous  organs. 

Regular  mental  work,  as  well  as  physical  work,  should  be 
limited  to  a  number  of  hours  per  day ;  and  these  should  be 
the  hours  before  late  in  the  afternoon,  when  the  maximum 
exhaustion  of  nervous  force  occurs.  Nervous  exhaustion 
from  mental  overwork  is  most  often  due  to  neglect  of  this 
rule;  and  the  brain-worker  should  limit  his  regular  day's 
work  to  a  reasonable  number  of  hours  per  day,  and  those 
when  the  brain  is  at  its  best.  Too  often  mental  overwork 
simply  means  such  long  days  at  intellectual  tasks  that  exercise, 
recreation,  and  sleep  are  neglected.  Sooner  or  later,  this 
means  the  inevitable  penalty  of  nervous  disturbance,  if  not 
serious  breakdown. 

There  come  times  in  the  lives  of  many  brain-workers  when 
some  important  work  demands  temporary  nervous  strain; 
but  the  man  who  is  wise  in  the  laws  of  hygiene  will  try  to  re- 


536  APPLIED  BIOLOGY 

duce  the  necessary  strain  to  the  minimum  and  to  follow  it 
with  as  much  recreation  as  possible. 

The  importance  of  fads  and  avocations  as  a  means  to  mental 
and  physical  recreation  is  great.  Every  man  and  woman 
should  cultivate  at  least  one  hobby.  Even  collecting  postage- 
stamps,  coins,  and  natural-history  specimens  may  be  made  an 
important  daily  relief  to  a  nervous  system  tired  by  the  reg- 
ular day's  work ;  but  best  of  all  avocations  are  those  which 
are  as  far  different  as  possible  from  the  regular  work,  e.g.,  a 
greenhouse,  a  garden,  or  a  work-shop  for  a  man  engaged  in 
mental  work. 

But  overwork  is  not  all  due  to  excessive  exercise  of  the 
nervous  organs  directly,  for  physical  work  may  lead  to  ner- 
vous disturbance.  This  is  obviously  due  to  the  fact  that  mus- 
cular contraction  occurs  only  as  the  result  of  nervous  action. 
Moreover,  there  may  be  the  added  effect  of  the  wearisome 
monotony  of  uninteresting  toil. 

The  close  relation  of  muscular  and  nervous  work  points 
to  the  important  hygienic  law  that  mental  work  should  not 
be  forced  after  physical  exhaustion,  or  physical  work  after 
becoming  mentally  tired.  The  time  for  hard  physical  exercise 
is  not  near  the  close  of  a  day  of  such  intense  mental  strain 
that  the  tired  nervous  system  seems  to  rebel  at  lashing  the 
muscles  into  action.  That  is  certainly  a  time  to  rest  or  rec- 
reate in  any  way  which  is  not  approached  as  an  unpleasant 
duty.  Conversely,  the  time  for  hard  study  is  not  at  the  close 
of  a  day  of  exhausting  physical  work.  Whether  mental  work 
may  safely  succeed  physical  activity,  or  vice  versa,  usually 
depends  upon  whether  one  finds  it  possible  to  take  up  the 
change  of  work  without  constantly  goading  oneself  against 
a  feeling  of  exhaustion.  Ambitious  workers  will  not  meet 
with  the  one  great  danger  in  this  advice;  namely,  that  of 
confusing  real  exhaustion  and  mere  laziness. 

460.  Sleep.  —  Probably  more  important  than  any  other 
rules  of  hygiene  are  those  concerning  sleep.  Loss  of  sleep 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      537 

does  far  more  damage  than  starvation.  Men  have  volun- 
tarily fasted  30,  40  and  even  50  days,  and  afterward  quickly 
regained  their  normal  weight  and  health;  but  it  is  certain 
that  a  normal  healthy  man  could  not  go  without  sleep  so 
long.  Sleep  is  a  period  of  rest,  repair,  and  growth,  and  is 
especially  important  for  growing  children.  Much  of  the 
physical  harm  to  children  in  crowded  tenements  is  due  to  a 
combination  of  late  retiring  to  rest,  uncomfortable  bedroom, 
disturbed  sleep,  and  early  rising.  Many  a  child  in  the  poorer 
regions  of  great  cities  is  anaemic,  nervous,  haggard  in  face, 
listless  in  school,  sluggish  at  play;  and  the  real  trouble  may 
be  that  he  is  not-  getting  enough  good  sound  sleep.  This  is 
likewise  true  of  some  adults,  who  should  average  seven  to 
nine  hours  of  sleep  in  twenty-four.  As  far  as  possible,  this 
should  be  taken  during  the  quietest  hours  (10  P.M.  to  6  A.M.) 
when  there  are  the  fewest  external  stimuli  tending  to  cause 
awakening. 

MUSCULAR  SYSTEM 

461.  Exercise  for  Health.  —  Those  whose  business  or 
pleasure  leads  them  to  sedentary  habits  of  life  need  to  con- 
sider most  seriously  the  question  of  physical  exercise.  The 
scientific  reason  for  exercise  is  to  be  found  in  the  coordina- 
tion of  muscular  activity  with  all  the  other  organs  of  the 
body,  rather  than  in  the  development  of  the  muscular  system 
itself.  In  short,  most  people  should  exercise  primarily  in 
order  to  get  reactions  of  the  digestive,  circulatory,  respira- 
tory, and  nervous  organs,  while  secondarily  and  incidentally 
they  may  develop  their  muscular  systems.  However,  it  is 
very  doubtful  in  the  opinion  of  many  qualified  physiologists 
whether  excessive  development  of  the  muscular  system  is 
best  for  the  general  health  of  those  who  are  not  professional 
athletes  or  laborers.  The  well-known  aphorism,  "  Mens 
sana  in  corpore  sano  "  (a  sound  mind  in  a  sound  body), 
means  that  giant  intellects  ought  to  be  located  in  healthy 


538  APPLIED  BIOLOGY 

bodies  in  which  all  the  functions  are  properly  coordinated, 
and  it  should  not  be  understood  to  mean  that  only  one  with 
the  muscles  of  a  champion  athlete  can  hope  to  do  great 
intellectual  work.  On  the  contrary,  it  is  a  remarkable  fact 
that  some  of  the  greatest  work  in  literature,  art,  and  science 
has  been  accomplished  by  men  and  women  who  suffered 
from  lifelong  physical  weakness.  In  such  examples  there 
is  hope  for  all  who  are  physically  weak  by  nature.  Athletic 
constitutions  commonly  originate  congenitally  and  not  in 
gymnasia. 

The  average  man  and  woman,  then,  should  exercise  for 
health,  deriving  it  from  renewed  activity  of  the  organs  that 
are  closely  coordinated  with  muscles,  and  from  rest  and 
recreation  for  the  brain.  This  reference  to  the  association 
of  exercise  and  recreation  is  important,  for  we  certainly 
derive  most  benefit  from  exercise  which  is  at  the  same  time 
pleasurable  and  recreative.  Herein  is  one  great  advantage 
of  many  forms  of  outdoor  exercise  over  gymnasium  work. 

462.  Excessive  exercise  is  not  beneficial  when  it  leads  to 
exhaustion ;  and  severe  over-strain  may  lead  to  injury  of  the 
heart,  blood-vessels,  or  lungs.  Athletic  enthusiasts  often 
answer  this  medical  criticism  against  certain  extra-strenuous 
games,  such  as  football  and  rowing,  by  claiming  that  the 
moral  gain  from  severe  athletic  contests  overbalances  the 
recognized  danger  of  great  physical  harm  from  excessive 
exhaustion.  That  there  is  moral  gain  worth  while  in  compell- 
ing tired  muscles  to  obey  to  the  point  of  exhaustion  is  ex- 
tremely doubtful;  and  those  who  are  fond  of  quoting  that 
"  Waterloo  was  won  on  the  playing-fields  of  Rugby  "  should 
re-read  history  and  note  the  victories  in  peace,  and  even  in 
battles  scarcely  less  strenuous  than  Waterloo,  and  by  men 
whose  moral  fiber  was  certainly  not  directly  traceable  to 
previous  athletic  training  on  any  school-playground.  Moral 
qualities  which  make  men  great  are  inherent,  not  originated 
by  any  one  form  of  activity ;  and  hence  we  are  not  justified 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING       589 

in  excusing  dangerously  excessive  exercise  because  one  or 
more  famous  generals  or  other  great  men  happened  to  play 
football,  or  some  other  game,  when  they  were  boys.  There 
is  no  scientifically  proved  moral  effect  of  physical  strain 
which  in  the  slightest  degree  militates  against  the  hygienic 
rule  that  exercise  for  health  should  never  be  carried  to  extreme 
exhaustion.  The  world  would  have  far  more  healthy  and 
efficient  men  if  this  rule  for  muscular  activity  were  more 
often  applied  both  in  work  and  in  play. 

H.   PHYSIOLOGICAL  EFFECTS   OF  STIMULANTS  AND 
NARCOTICS  * 

463.  Introduction.  —  Man  has   long  been   more   or  less 
accustomed  to  take  into  the  body  certain  substances  (alcohol, 
tobacco,  tea,  coffee,  certain  drugs,  etc.)  which  are  not  prop- 
erly classed  with  the  ordinary  food  materials,  for  their  value 
as  sources  of  energy  and  materials  for  repair  is  so  slight  as  to 
be  negligible.     Ail  these  substances  are  conveniently  grouped 
under    the    heading    "  Stimulants    and    Narcotics,"    which 
indicates  that  their  action  in  the  human  body  is  either  to 
excite  or  stimulate  greater  activity  of  certain  organs,  or  to 
reduce  their  activity  and  tend  to  produce  stupor  or  sleep 
(narcosis).     Both  the  exciting  action  of  stimulants  and  the 
quieting  effect  of  narcotics  are  pleasurable  to  most  people, 
and  it  is  solely  for  this  peculiar  pleasure  that  mankind  has 
adopted   the   habit   of  using  the  various  substances  which 
afford  stimulating  and  narcotic  effects. 

464.  Are    Stimulants    and    Narcotics    Needed  ?  —  It    is 
interesting  to  note  that  no  animal  naturally  makes  use  of 
any  of  the  stimulants  and  narcotics ;    and  hence  it  is  often 
argued  that  the  human  species  ought   to  be  natural  and 
avoid  them.     However,  this  is  a  rather  weak  argument,  for 


*  To  TEACHERS:   See  "Teachers'  Manual"  for  notes  concerning  the  use 
of  this  section. 


540  APPLIED  BIOLOGY 

in  many  other  ways  man  has  ceased  to  be  natural  (e.g.,  cook- 
ing food  is  certainly  unnatural  for  animals),  and  with  advan- 
tage to  himself.  Clearly  the  use  of  stimulants  and  nar- 
cotics must  be  judged  by  their  good  or  bad  effects  upon  men, 
and  not  rejected  simply  because  animals  do  not  use  them. 
The  experience  of  animals  indicates  that  man  does  not  abso- 
lutely need  stimulants  and  narcotics ;  but  it  has  no  bearing 
whatever  on  the  question  of  whether  man  may  or  may  not 
profitably  make  use  of  such  substances.  This  will  be  dis- 
cussed at  various  places  in  the  following  sections.  The 
general  conclusion  is  that  under  some  conditions  there  is  need 
of  stimulation,  or  of  quieting  organs  by  narcotics ;  but  that, 
on  the  whole,  stimulants  and  narcotics  are  easily  used  in 
excess. 

465.  Examples   of   Stimulants   and   Narcotics.  —  Alcohol 
in  small  quantities  is  a  stimulant  which  increases  the  activi- 
ties of  many  organs.     It  is  well   known   that   alcohol   in 
large  quantities  produces  a  narcotic  effect,  leading  to  the 
complete  stupor  of  intoxication.     Opium  is  well  known  as 
a  powerful  narcotic  which  quiets  active  organs,  and  in  large 
doses  leads  to  a  fatal  sleep.     Most  users  of  tobacco  in  any 
form  claim  that  it  has  a  soothing  effect,  i.e.,  is  a  narcotic. 
Tea  and  coffee  contain  substances  which  are  usually  stimulat- 
ing to  most  persons.     Many  drugs  used  by  physicians  (e.g., 
strychnine,  nitro-glycerine)   are    powerful  stimulants,    and 
are  given  in  exceedingly  small  quantities.     When  powerful 
narcotics   are   demanded   as   relievers   of   pain,    physicians 
commonly  use  opium  and  its  products  (laudanum,  morphine). 

466.  Alcohol  and  Common  Alcoholic  Fluids.  —  The  forma- 
tion of  alcohol  from  sugar  has  already  been  described  in 
§§  250, 251,  which  deal  with  fermentation  caused  by  the  yeast- 
plant.     Practically  any  natural  substance  which   contains 
starch  or  sugar  may  undergo  fermentation.      Thus  juices 
expressed  from  grapes,   apples,   and  other  fruits,   and  the 
carbohydrates  in  grains  of  rye,  corn,  and  barley,  and  in 


BIOLOGY  APPLIED   TO  HEALTHFUL   LIVING      541 

potatoes,  are  commonly  used  in  producing  alcoholic  liquors, 
of  which  the  chief  varieties  are  mentioned  below. 

Malt  liquors  (beer,  ale,  and  porter)  are  made  from  malt, 
which  is  generally  sprouted  barley  grains.  This  is  ground  in 
water,  and  allowed  to  ferment.  Hops  are  added  to  give  a 
bitter  flavor.  Such  a  fermented  liquor  consists  chiefly  of 
water,  1  to  8  per  cent  alcohol,  and  small  quantities  of  other 
substances  derived  from  the  grains  used. 

Wines  are  juices  of  grapes  which  have  fermented  and  pro- 
duced 6  to  12  per  cent  of  alcohol.  Some  wines  are  stronger 
because  brandy  or  strong  alcohol  has  been  added  when 
bottling. 

Distilled  liquors  (whisky,  gin,  brandy,  rum)  contain  30 
to  50  per  cent  alcohol,  and  are  made  thus  strong  by  distilling 
the  fermented  fluids  (water  with  rye,  corn,  oats,  or  potatoes 
for  gin  and  whisky ;  molasses  in  water  for  rum ;  wines  for 
brandy) .  Various  flavoring  and  coloring  materials  are  added 
to  the  distilled  liquors.  They  differ  essentially  only  in 
color,  flavor,  and  proportions  of  alcohol.  Some  of  the  sub- 
stances used  to  color  and  flavor  are  harmful,  but  are  used  in 
such  small  quantities  that  the  alcohol  is  chiefly  responsible 
for  the  physiological  injury  done  by  the  distilled  liquors. 

Alcohol  in  a  more  or  less  pure  state  can  be  made  by  re- 
distilling and  otherwise  purifying  any  fluid  in  which  fermen- 
tation has  occurred.  Since  distilled  liquors  are  nearly  half 
alcohol,  it  is  easily  obtained  from  them.  The  grain  alcohol 
of  commerce  is  usually  from  91  to  95  per  cent  pure ;  i.e.,  it 
contains  5  to  9  per  cent  of  water.  A  special  quality  for 
scientific  purposes  is  about  99  per  cent  pure,  and  is  very  ex- 
pensive to  make. 

Commercial  alcohol  is  usually  called  "  grain-alcohol,"  or, 
in  chemical  terms,  ethyl  alcohol.  Wood-alcohol,  or  methyl 
alcohol,  is  commonly  made  from  wood.  In  all  the  discussions 
in  this  lesson,  the  word  "  alcohol "  is  used  to  mean  grain  or 
ethyl  alcohol,  for  this  is  the  characteristic  constituent  of 


542  APPLIED  BIOLOGY 

the  alcoholic  liquors  whose  physiological  effects  are  under 
consideration.  We  shall  see  that  the  effect  of  alcoholic 
liquors  is  largely  in  proportion  to  the  amount  of  contained 
alcohol,  and  so  it  is  justifiable  and  convenient  to  deal  directly 
with  the  effects  of  alcohol  and  neglect,  temporarily,  the  minor 
fact  that  alcohol,  as  commonly  taken  in  wine,  beer,  whisky, 
etc.,  is  diluted  with  water  and  variously  flavored. 

467.  Is  Alcohol  a  Poison  ?  —  In  popular  usage,  the  word 
"  poison  "  is  associated  with  such  powerful  substances  as 
arsenic,  strychnine,  snake-venom,  and  others  which,  when 
introduced  into  the  human  body,  produce  marked  and  even 
fatal  disturbances.  In  scientific  usage  the  term  is  applied 
to  many  substances  which  cause  demonstrable  disturbance  of 
any  function  of  the  body.  There  is  no  substance  which  is 
always  a  poison,  for  even  strychnine  and  ricin  may  be  diluted 
so  as  to  produce  no  noticeable  disturbance.  A  cup  of  coffee 
is  not  poisonous  to  an  average  adult,  and  yet  it  contains  a 
greatly  diluted  dose  of  caffein,  which  in  large  amounts  is  a 
poison.  Tea,  coffee,  ginger,  pepper,  and  many  other  things 
taken  with  food  contain  small  quantities  of  substances  which 
in  large  amounts  are  poisons.  Even  common  salt  in  very 
large  quantities  has  proved  a  fatal  poison.  Evidently  the 
word  "  poison  "  has  a  relative  significance,  and  involves  the 
quantity.  In  general,  we  apply  it  only  to  substances  which 
in  very  limited  quantity  are  harmful.  The  question,  then, 
"  Is  alcohol  a  poison?  "  can  be  answered  only  by  reference 
to  the  amount  of  alcohol  and  to  the  constitution  of  the  indi- 
vidual who  drinks  it.  That  alcohol  in  large  and  intoxicating 
doses  has  proved  fatally  poisonous  is  well  known,  and  that 
it  commonly  produces  profound  disturbances  of  various 
organs  when  used  excessively  and  habitually  is  also  common 
knowledge ;  but  whether  alcohol  in  very  small  quantities  is 
a  poison,  is  a  difficult  scientific  question  which  only  physi- 
ologists can  answer  by  experimental  studies  made  with  ani- 
mals and  men. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      543 

A  liter  (nearly  a  quart)  of  whisky,  gin,  or  rum  given  in  one 
dose  would  kill  any  animal  weighing  67  kilograms.  (How 
many  pounds?)  Evidently  alcohol  in  large  quantities  is  a 
poison.  However,  even  a  victim  of  the  alcoholic  habit 
would  not  drink  a  quart  of  whisky  within  a  short  time. 
The  question  is  whether  in  the  smaller  amounts,  such  as  are 
commonly  used  by  drinkers,  alcohol  should  be  classed  as  a 
poison.  The  next  four  sections  discuss  this  question  with 
reference  to  the  organs  of  digestion,  circulation,  and  respira- 
tion, and  the  nervous  system ;  and  it  is  pointed  out  that  it 
is  impossible  to  show  by  scientific  methods  that  ordinary 
small  amounts  of  alcohol  produce  effects  comparable  to  those 
of  the  substances  which  druggists  label  "  poison."  Hence  it 
must  be  concluded  that,  so  far  as  we  now  know,  alcohol  in 
small  amounts  is  not  harmful  enough  to  warrant  labeling  it 
"  poison."  We  do  not  so  label  common  salt,  although  a 
strong  solution  taken  into  the  stomach  has  caused  death; 
and  while  coffee  contains  a  dangerous  poison,  a  pot  of  the 
beverage  should  not  be  labeled  as  dangerous.  Most  people 
would  be  misled  by  such  a  label,  for  they  know  well  that  in 
ordinary  quantities  common  salt  and  coffee  produce  no  symp- 
toms of  poisoning.  Likewise,  in  very  limited  quantity 
alcohol  is  not  a  poison  in  the  sense  that  we  understand 
various  drugs  to  be  poisons. 

468.  Effect  of  Alcoholic  Liquors  on  Digestion.  —  When 
taken  into  the  stomach,  alcoholic  fluids  cause  a  marked  in- 
crease in  the  flow  of  gastric  juice  from  the  glands  of  the 
stomach  wall.  There  is  also  an  increase  in  the  amount  of  the 
constituents  of  gastric  juice :  namely,  pepsin  and  hydro- 
chloric acid.  Wine,  alcohol,  beer,  whisky,  brandy,  and 
wines  —  all  stimulate  the  gastric  glands  in  this  way.  The 
alcohol  quickly  leaves  the  stomach,  being  absorbed  into  the 
blood,  leaving  the  gastric  juice  in  concentrated  form. 
Whether  such  an  effect  of  alcoholic  fluids  upon  gastric  secre- 
tion is  directly  harmful  or  not  seems  to  depend  upon  the 


544  APPLIED  BIOLOGY 

amount  of  alcohol  present.  Thus  strong  beverages,  like 
brandy,  gin,  and  whisky,  with  40  to  50  per  cent  of  aleohol, 
retard  the  digestive  action  of  pepsin  on  proteids;  but  this 
effect  depends  upon  the  amount  of  alcohol  taken,  the  amount 
of  food  in  the  stomach,  the  strength  of  the  gastric  juice,  and 
the  health  of  individuals.  Hence  it  is  impossible  to  lay  down 
any  general  rule  as  to  the  minimum  quantity  of  alcohol  which 
will  harmfully  affect  digestion  in  the  stomach.  It  is  certain, 
however,  that  intoxicating  doses  do  impede  gastric  digestion 
even  in  healthy  individuals. 

But  it  should  be  emphatically  stated  that  the  effect  of 
alcoholic  drinks  upon  digestion  is  not  solely  due  to  the  amount 
of  alcohol  in  them.  Thus  sherry  wine  with  20  per  cent  of 
alcohol  retards  digestion  much  more  than  does  an  equal 
quantity  of  20  per  cent  pure  alcohol.  Large  amounts  of 
claret  wines  have  a  similar,  but  less,  effect.  The  same  is 
true  of  ale,  beer,  and  other  malt  liquors.  When  any  of  the 
wines  and  malt  liquors  are  used  freely  with  meals,  there  is 
likely  to  be  a  considerable  retardation  of  the  digestive  pro- 
cesses. 

Concerning  the  effect  of  moderate  amounts  of  alcoholic 
fluids  upon  gastric  digestion,  it  appears  from  experiments 
made  by  competent  investigators  that  the  greater  secretion 
of  gastric  juice  is  counterbalanced  by  the  retarding  effect; 
and  hence,  as  a  rule,  there  is  no  reason  for  or  against  using 
small  amounts  with  meals,  so  far  as  the  effect  on  gastric 
digestion  is  concerned.  But  we  shall  see  later  (§§471,  473) 
that  alcohol  has  much  more  decided  effects  upon  other 
organs  which  help  us  to  decide  for  or  against  its  use. 

469.  Effect  of  Alcohol  on  Blood-System.  —  Small  quan- 
tities, as  used  by  physicians  in  cases  of  great  depression  of  the 
heart,  stimulate  that  organ  reflexly  through  the  nervous 
system.  In  large  and  intoxicating  quantities  alcohol  is  a 
hrect  and  powerful  depressant,  weakening  the  beat,  distend- 
ing the  cavities,  and  diminishing  the  pumping  of  blood. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      545 

Herein  is  the  scientific  reason  for  the  remark  in  §  353  concern- 
ing the  use  of  alcoholic  fluids  for  snake-bites. 

It  is  well  known  that  alcohol  in  small  amount  causes  flush- 
ing of  the  face  and  a  sense  of  heat  over  the  skin.  This  is 
due  to  the  dilation  of  blood-vessels. 

The  combined  effect  of  large  quantities  of  alcohol  on  heart 
and  blood-vessels  is  to  lower  the  pressure  of  the  blood. 

Whether  these  effects  are  harmful  or  not  depends  upon 
conditions,  especially  of  organs  other  than  the  blood-system. 
It  is  a  significant  fact  that  the  modern  physician  is  careful  in 
prescribing  alcohol,  even  in  moderate  doses,  when  he  desires 
stimulating  effects  on  the  heart  and  blood-vessels;  for  the 
effects  on  other  organs  may  more  than  counterbalance  any 
possible  good  done  to  the  organs  of  circulation. 

470.  Alcohol  and  Respiration.  —  For  a  short  time  after 
drinking  alcoholic  fluids  there  is  an  increase  in  the  rate  of 
respiration.     This  is  probably  due  to  an  increased  loss  of 
heat  from  the  dilated  blood-vessels  of  the  skin  (§447).     In 
other  words,  the  respiratory  organs  must  work  faster  in  order 
to  supply  oxygen  for  the  increased  internal  oxidation  needed 
to  supply  heat  in  place  of  that  lost.     Such  a  chain  of  events 
leads  many  physicians  to  question  seriously  whether  good 
or  harm  will  come  from  a  dose  of  alcohol,  as  in  pneumonia, 
with  weakened  heart  and  lungs  already  congested  with  blood. 
At  any  rate,  it  is  exceedingly  doubtful  whether  in  conditions 
of  health  any  useful  purpose  is  served  by  increasing  respira- 
tion by  means  of  alcohol. 

471.  Alcohol  and  Nervous  Organs.  —  The  general  influence 
of  large  amounts  of  alcohol  on  the  nervous  system  is  well 
known  to  all  who  have  observed  the  actions  of  drunken  men. 

Large  quantities  of  alcohol  lessen  all  mental  activities. 
Careful  experiments  have  shown  that  even  a  pint  of  wine 
diminishes  acuteness  of  smell  and  touch  and  interferes  with 
the  power  of  the  eye  to  estimate  measurements.  Psycholo- 
gists have  failed  to  prove  that  alcohol  increases  the  quantity 

2N 


546  APPLIED  BIOLOGY 

and  vigor  of  mental  operations ;  on  the  contrary  even  small 
doses  tend  to  lessen  reasoning  power.  Larger  quantities 
affect  the  power  of  attention,  judgment,  and  reason,  render 
the  senses  less  acute,  and  exert  an  anaesthetic  action  which 
leads  to  the  sleep  characteristic  of  intoxication.  Study  of 
many  such  conditions  leads  eminent  experts  on  drugs  to  the 
opinion  that,  even  in  moderate  quantities,  alcohol  tends  to 
have  a  sedative  or  narcotic  action  on  the  brain.  In  connec- 
tion with  this  statement,  may  be  cited  the  undoubted  fact 
that  alcohol  regularly  used  during  the  day's  work  diminishes 
the  amount  and  quality  of  work  done.  This  has  been  ex- 
perimentally proved  by  tests  with  type-setters  and  others 
whose  work  is  of  such  a  nature  that  it  is  easy  to  compute  both 
speed  and  accuracy.  Those  who  command  armies  and  large 
groups  of  men  engaged  in  physical  labor  agree  that  the  use  of 
alcohol  during  work  decreases  effectiveness.  This  is  prob- 
ably because  of  the  sedative  action  above  mentioned. 

It  has  not  yet  been  possible  for  investigators  to  collect 
and  collate  the  facts  as  to  the  effect  of  small  amounts  of 
alcoholic  liquors  upon  large  numbers  of  brain-workers  (law- 
yers, teachers,  clergymen,  business  men,  physicians),  for 
the  reason  that  many  will  not  or  cannot  answer  questions 
concerning  their  own  experience  with  alcoholic  drinks. 
One  list  of  892  brain-workers  in  the  United  States  showed 
167  total  abstainers,  579  occasional  drinkers,  and  146  mod- 
erate drinkers.  A  large  number  of  the  moderate  drinkers 
expressed  the  opinion  that  the  use  of  alcoholic  drinks  gives 
bad  results  as  stimulants  to  mental  work,  and  many  also 
stated  that  they  used  alcohol  from  habit,  and  with  no  expec- 
tation of  being  enabled  to  do  more  or  better  mental  work. 

In  spite  of  the  fact  that  we  cannot  say  definitely  how  little 
alcohol  will  seriously  interfere  with  the  normal  functions  of 
the  nervous  system,  it  is  clear  that  the  brain-worker  acts 
most  wisely  by  avoiding  alcohol  during  his  mental  work. 
Whether  he  will  gain  or  not  by  avoiding  alcohol  altogether 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      547 

cannot  yet  be  demonstrated,  because  the  possible  accumula- 
tive effects  of  small  doses  of  alcohol  are  still  unknown  to  science. 

472.  Nutritive  Value  of  Alcohol.  —  "  Is  alcohol  a  food  ?  " 
The  answer  depends  upon  what  we  understand   by  food. 
It  is  not  food  in  the  sense  that  bread  and  meat  are  foods,  for 
it  cannot  support  life.     It  lacks  the  nitrogen  and  necessary 
mineral  elements  for  growth  and  repair.       But  it  may,  in 
small  quantities,  take  the  place  of  foods  used  for  fuel  or 
energy,  for  some  is  oxidized  in  the  human  body. 

However,  it  is  of  little  moment  that  alcohol  has  a  slight 
food  value,  for  against  it  there  are  great  objections  as  fol- 
lows :  (1)  Only  srnall  amounts  can  be  used  as  food,  and  it  is 
easy  to  overestimate  the  amount  which  is  safe.  (2)  There 
is  a  peculiar  tendency  to  excessive  and  habitual  use.  (3)  Its 
action  as  a  harmful  drug  may  overbalance  its  value  as  food. 
(4)  It  is  a  very  expensive  food  as  compared  with  carbohy- 
drates and  fats,  which  can  supply  equivalent  energy. 

Such  grave  objections  to  alcohol  in  any  form  make  it 
necessary  to  regard  its  food  value  as  of  little  importance. 
One  of  the  best  of  advanced  books  on  physiology  well  sum- 
marizes the  whole  matter  as  follows :  "  Only  in  very  excep- 
tional cases  can  alcohol  have  any  practical  importance  as  a 
nutriment.  It  is  especially  in  the  case  of  acute  diseases 
accompanied  by  diminished  digestive  power  that  alcohol 
seems  to  serve  as  a  valuable  nutriment." 

After  all,  no  one  regularly  uses  alcohol  as  food,  but  rather 
for  its  peculiar  taste  and  stimulating  effect.  The  use  or 
disuse  of  alcohol  must  depend  upon  the  answer  to  the  ques- 
tion, "Is  the  stimulating  effect  of  alcohol  injurious?" 
Except  in  small  quantities  it  certainly  is  an  injurious  stimulant; 
and  no  one  can  safely  estimate  the  quantity  which  may  not  lead 
to  accumulated  effects,  or  to  habits  of  excess. 

473.  Disease    Effects    of    Alcohol.  —  Many    organs    are 
known  to  become  diseased  as  the  result  of  long-continued 
excessive  use   of  alcoholic   drinks.     Liver,   kidneys,   heart, 


548  APPLIED  BIOLOGY 

blood-vessels,  and  nervous  organs  are  frequently  involved  in 
disease  changes.  All  medical  men  recognize  that  alcoholic 
intemperance  leads  to  an  immense  amount  of  sickness.  A 
large  number  of  deaths  are  due  to  the  diseases  known  as 
chronic  alcoholism  and  delirium  tremens;  and  many  cases 
of  Bright's  disease  of  the  kidneys,  paralysis,  pneumonia, 
tuberculosis,  and  other  diseases  are  believed  by  eminent 
physicians  to  have  been  hastened  to  a  fatal  issue  by  the  pre- 
vious use  of  alcohol. 

It  should  be  noted  that  no  reputable  physician  claims  that 
even  excessive  use  of  alcohol  will  always  lead  to  diseased 
conditions.  There  are  exceptional  individuals  who  are 
almost  constantly  intoxicated,  and  yet  show  no  external 
evidences  of  diseased  organs.  However,  no  sane  person  who 
has  learned  of  the  great  liability  of  excessive  drinkers  to 
diseases  will  care  to  take  his  chances  of  being  one  of  the  few 
who  appear  to  escape  the  most  serious  consequences.  More- 
over, it  should  be  noted  that  in  recent  years  there  have  been 
found  many  cases  of  diseases  in  "  moderate  drinkers  "  which 
are  probably  due  to  long  use  of  alcoholic  liquors. 

It  is  well  known  that  the  excessive  use  of  alcohol  leads  to 
obesity \  or  storing  of  fat.  This  is  most  dangerous  when  in 
the  muscles  of  the  heart.  Gout  is  often,  not  always,  caused 
by  alcoholic  liquors. 

474.  "  Pure  "  Alcoholic  Beverages.  —  Manufacturers  of 
alcoholic  liquors  often  advertise  that  their  products  have 
been  "  purified  "  so  that  they  have  no  poisonous  action. 
This  is  an  absolutely  false  claim,  for  scientific  studies  have 
shown  that  while  the  so-called  "  impurities  "  in  alcoholic 
drinks  are  poisonous,  they  are  present  in  such  small  amounts 
that  their  effect  is  slight,  and  that  the  poisonous  effect  of 
alcoholic  liquors  is  chiefly  due  to  the  alcohol  they  contain. 
Even  absinthe  and  other  highly  flavored  French  liqueurs, 
containing  extracts  of  wormwood,  anise,  and  other  aromatic 
herbs,  certainly  owe  most  of  their  decidedly  poisonous  action 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      549 

to  their  large  amount  of  alcohol  (50  to  80  per  cent).  Alcohol 
in  large  quantities  has  been  demonstrated  to  be  poisonous 
enough  to  account  for  most  of  the  physiological  evils 
ascribed  to  alcoholic  drinks.  So  long  as  people  will  drink 
alcoholic  fluids,  there  should  be  "  pure  food  "  laws  aimed  at 
making  them  as  free  from  harmful  substances  as  possible; 
but  no  one  should  be  deceived  by  claims  that  a  given  brand 
of  liquor  is  harmless.  Whenever  alcohol  is  present  in  con- 
siderable amount,  there  is  a  substance  which,  in  quantity 
varying  with  individuals  and  conditions,  is  certain  to  be 
harmful  in  its  effect  upon  the  essential  life-processes.  Even 
the  "  best  "  alcoholic  beverages  have  the  power  to  do  this. 

The  common  opinion  that  the  cheap  artificial  whisky  sold 
in  some  saloons  for  three  cents  a  glass  is  especially  injurious 
to  health  as  compared  with  "  high  grade  "  natural  whisky 
made  from  corn  and  rye  is  not  supported  by  chemical 
analysis.  This  cheap  whisky  consists  of  30  to  50  per  cent 
alcohol  with  caramel,  sugar,  and  flavoring  essences.  Its 
harmfulness  depends  chiefly  upon  the  alcohol  contained. 
Perhaps  the  chief  reason  why  such  cheap  liquors  appear 
harmful  is  that  the  low  price  leads  to  the  use  of  more  alcohol. 
Certain  it  is,  however,  that  the  chief  danger  in  cheap  whisky, 
and  in  all  whisky,  lies  in  the  30  to  50  per  cent  of  alcohol  which 
it  contains. 

475.  Alcoholic  "  Temperance  Drinks." — Very  many  people 
who  hold  strictly  to  temperance  principles  are  unaware  that 
many  fluids  sold  at  drug-stores  contain  large  amounts  of 
alcohol.  In  general,  all  "  tonics,"  "  bitters,"  "  malt-ex- 
tracts," "  celery  compounds,"  and  other  similar  fluids  adver- 
tised as  givers  of  strength,  vigor,  etc.,  contain  alcohol. 
Certain  much-advertised  medicines  called  "  sarsaparilla " 
contain  at  least  25  per  cent  of  alcohol.  Many  "  bitters  " 
and  "  tonics  "  contain  from  15  to  45  per  cent  alcohol.-  Some 
of  these  are  advertised  as  containing  no  alcohol,  or  as  "  tem- 
perance "  medicines.  According  to  the  labels  on  the  bottles, 


550  APPLIED  BIOLOGY 

doses  as  large  as  a  wineglassful  four  times  daily  are  sometimes 
advised.    This  means  a  large  amount  of  alcohol. 

Root-beers,  ginger  ale,  and  fermented  milk  contain  very 
small  amounts  of  alcohol,  usually  less  than  one  per  cent. 
So-called  "  sweet  cider  "  sold  by  all  dealers  may  have  more 
alcohol  than  the  average  beer,  and  frequently  contains  0.2 
to  3.5  per  cent.  "  Hard  "  or  fermented  cider  contains  4  to  8 
per  cent  of  alcohol,  and  therefore  compares  with  mild  wines 
and  strong  beer. 

476.  Alcohol   as    Medicine.  —  Concerning   the   value   of 
alcohol  in  treating  diseases,  great  authorities  on  medicine  do 
not  yet  agree.      There  are  many  eminent  physicians  who 
never  prescribe  it,  but  prefer  to  use  drugs  whose  stimulating 
action  is  more  definite  and  certain  than  that  of  alcohol. 
Other  equally  eminent  doctors  hold  that  alcohol  is  of  great 
value  in  certain  acute  diseases  where  there  is  a  tendency 
toward  general  and  heart  weakness. 

477.  Alcohol  and   Growth.  —  It  is  universally  admitted 
by  physiologists  that  all  alcoholic  drinks  are  deleterious  to 
growing  individuals,  and  this  means  the  first  eighteen  or 
twenty  years  of  life.     It  is  absolutely  pernicious  to  young 
children.     And  in  addition  to  its  direct  physiological  injury 
to  young  people,  there  is  the  oft-demonstrated  tendency  to 
excessive  use.     The  vast  majority  of  habitual  drinkers  of 
alcholic  liquors  begin  in  early  life. 

478.  Summary  of  Effects  of  Alcohol.  —  Professor  Atwater, 
the  famous  chemist,  who  contributed  much  to  our  knowl- 
edge of    foods  and  their  uses,  wrote  that  in  his  personal 
opinion   "  people  in  health,   and  especially  young  people, 
act  most  wisely  in  abstaining  from  alcoholic  beverages." 

A  committee  of  five  prominent  American  physiologists 
has  thus  summarized  the  facts  regarding  the  use  of  alcohol : 

(1)  While  in  moderate  quantities  beer  and  wine  may  be  in 
a  certain  sense  a  food,  they  are  a  very  imperfect  and  expen- 
sive kind  of  food,  and  are  seldom  used  for  food  purposes; 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      551 

(2)  they  are  not  needed  by  young  and  healthy  persons,  and 
are  dangerous  to  them  in  so  far  as  they  tend  to  create  a 
habit ;  (3)  in  certain  cases  of  disease  and  weakness  they  are 
useful  in  quantities  to  be  prescribed  by  physicians ;  (4)  when 
taken  habitually,  it  should  be  only  at  meals ;  and,  as  a  rule, 
only  with  the  last  meal  of  the  day,  or  soon  after  it ;  (5)  alco- 
holic drinks  of  all  kinds  are  worse  than  useless  to  prevent 
fatigue  or  the  effects  of  cold,  although  they  may  at  rare  times 
be  useful  as  restoratives;  (6)  they  are  almost  always  a 
useless  expense ;  (7)  their  use  in  excess  is  the  cause  of  much 
disease,  suffering,  and  poverty,  and  of  many  crimes;  (8) 
excessive  use  is  sometimes  the  result,  rather  than  the  cause, 
of  disease. 

479.  Effects  of  Tobacco.  —  Concerning  the  effect  of 
tobacco  in  its  various  forms  upon  health,  there  has  been 
much  discussion.  As  is  well  known,  the  stem  and  leaves  of 
the  tobacco  plant  contain  a  poisonous  substance  known  as 
nicotine,  which,  in  concentrated  doses,  quickly  kills  small 
animals.  However,  this  proves  nothing  regarding  the  effect 
of  smoking  tobacco,  or  of  the  disgusting  habit  of  chewing  it, 
which  is  now  almost  unknown  among  the  better  classes  of 
people ;  for  in  both  of  these  ways  of  using  tobacco  the  nico- 
tine is  exceedingly  diluted,  as  is  the  poison  found  in  tea  and 
coffee.  The  result  is  that  the  effects  of  tobacco  are  not 
marked,  and  so  even  physicians  are  not  always  certain  as  to 
its  influence  upon  their  patients.  The  best  established 
knowledge  we  now  have  is  that  indigestion,  irritation  of  the 
respiratory  organs,  and  heart  and  nervous  disturbance 
may  in  some  people  result  from  the  use  of  tobacco,  while 
others  show  no  apparent  effect. 

All  this  refers  to  healthy  adult  men,  for  all  medical  authori- 
ties agree  that  tobacco  is  always  harmful  to  growing  boys, 
and  interferes  with  their  physical  and  mental  development. 
All  schoolboys  know  how  rigidly  most  athletic  trainers 
forbid  the  use  of  tobacco  by  those  in  training. 


552  APPLIED  BIOLOGY 

The  direct  effect  of  tobacco  is  narcotic,  and  many  smokers 
say  that  it  "  soothes  their  nerves."  It  is  very  doubtful 
whether  the  nerves  would  need  "  soothing  "  if  the  tobacco 
habit  had  not  been  established.  Opium  also  has  such  an 
effect ;  but  it  is  well  known  that  the  craving  for  the  soothing 
is  the  result  of  the  established  habit,  and  those  who  never 
used  opium  do  not  need  to  be  soothed  by  that  drug.  Like- 
wise those  who  have  never  acquired  the  tobacco  habit  appear 
to  have  no  need  for  its  narcotic  or  soothing  effect.  The 
difference  between  the  opium  and  tobacco  habits  is  in  degree, 
not  in  kind.  Both  create  a  demand  or  craving  for  their 
peculiar  narcotic  effects. 

Some  few  people  have  their  eyes  seriously  affected  by  even 
small  amounts  of  tobacco,  while  many  find  that  tobacco 
smoke  irritates  the  eye-membranes  and  causes  some  blurring 
of  vision. 

The  whole  physiological  truth  about  tobacco  so  far  as  now 
known  is  that :  (1)  no  one  needs  it  except  to  satisfy  an  estab- 
lished habit ;  (2)  many  adults  are  injured  by  it,  and  no  one 
knows  just  how  much  will  do  harm  to  a  particular  person; 

(3)  some  adults  are  apparently  not  harmed  by  limited  use ; 

(4)  it  is  decidedly  injurious  to  growing  boys ;   (5)  those  who 
avoid  establishing  the  habit  in  youth  do  not  as  a  rule  care  to 
learn  later,  for  there  are  no  physiological  reasons  why  any  one 
should  deliberately  set  out  to  learn  the  use  of  tobacco  in  any 
form. 

480.  Effects  of  Tea,  Coffee,  and  Cocoa.  —  The  first  two 
are  most  important  because  they  are  so  widely  used  as 
beverages.  It  is  now  well  known  to  physicians  that  many 
people  drink  too  much  tea  and  coffee,  and  that  temperance 
is  needed  in  use  of  these  beverages  no  less  than  with  alcoholic 
drinks.  Their  stimulating  effect  is  due  to  the  presence  of  a 
powerful  drug  (caffein),  which  has  a  stimulating  action  on 
the  nervous  system.  Nervousness,  insomnia,  headache, 
and  indigestion  are  common  symptoms  arising  from  exces- 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      553 

sive  use  of  tea  or  coffee ;  and  disturbances  of  other  organs  may 
follow.  One  who  uses  even  small  amounts  of  tea  and  coffee 
and  who  does  not  "  feel  well "  for  no  apparent  reason,  should 
try  the  effect  of  complete  abstaining  for  a  few  days  occa- 
sionally. By  so  experimenting  with  themselves,  many  people 
have  learned  that  tea  and  coffee  harm  them.  Other  people 
are  certainly  benefited  by  a  limited  use  of  these  beverages. 

Tea  and  coffee  should  never  be  given  to  young  children. 
They  may  be  easily  harmed  by  such  stimulants. 

Cocoa  and  chocolate  are  made  from  the  seeds  of  the  cacao- 
or  chocolate-tree,  a  native  of  tropical  America.  The  seeds 
are  rich  with  50  per  cent  of  fat,  some  of  which  is  extracted  in 
the  process  of  making  commercial  cocoa  or  chocolate.  Both 
of  these  contain  a  substance  similar  to  the  caffein  of  tea  and 
coffee,  but  milder  in  its  effects.  However,  one  who  has 
difficulty  from  the  use  of  tea  and  coffee  will  do  well  to  experi- 
ment with  himself  and  thus  learn  whether  chocolate  or 
cocoa  produces  marked  effects. 

481.  Effect  of  Narcotic  Drugs.  —  We  are  here  concerned 
with  the  effect  of  such  drugs  as  opium,  morphine,  cocaine, 
laudanum,  chloroform,  chloral  hydrate,  and  various  patent 
or  secret  preparations,  all  of  which  are  habitually  used  by 
some  people.  In  most  cases,  such  drugs  are  used  first  to 
narcotize  the  nerves  and  thus  relieve  pain,  and  their  frequent 
use  finally  becomes  a  habit  even  more  powerful  than  the 
alcoholic  habit.  It  is  unnecessary  to  go  into  an  extensive 
discussion  of  narcotic  drugs,  for  most  intelligent  people  now 
understand  that  only  on  a  physician's  advice  is  it  safe  to  use 
any  drug  to  relieve  pain ;  and  also  that  no  drug  should  be 
used  so  frequently  as  to  offer  the  grave  risk  of  establishing  a 
habit. 

It  should  be  noted  that  the  narcotic  drugs  do  not  cure 
diseases.  For  example,  patent  headache  powders  and  pills 
simply  dull  the  sensory  nerves.  The  headache  may  be  due 
to  a  disordered  stomach,  eye-strain,  constipation,  or  other 


554  APPLIED  BIOLOGY 

causes ;  and  hence  the  narcotics  give  only  temporary  relief. 
Obviously,  it  is  wiser  to  consult  a  doctor,  who  may  be  able 
to  find  the  cause  and  prescribe  treatment, 

The  remarks  made  above  concerning  narcotic  drugs  might 
well  be  applied  to  habitual  taking  of  any  kind  of  medicine 
without  a  physician's  advice.  An  immense  amount  of  harm 
is  done  by  the  thousands  of  patent  medicines. 

IH.    BACTERIOLOGY  APPLIED  TO  HUMAN  HEALTH 

482.  Bacteriology  and  Health.  —  The  principles  of  bac- 
teriology which  have  been  discovered  within  the  past  thirty 
years  are  not  only  of  interest  in  connection  with  the  cause 
and  cure  of  many  diseases,  as  stated  in  §  259,  but  are  also  of 
much  greater  importance  in  that  they  are  capable  of  being 
applied  so  as  to  maintain  health.     The  relation  of  bacteri- 
ology to  hygiene  is  already  a  vast  subject,  and  we  can  take 
time  for  only  a  few  of  the  most  important  points.     We  shall 
consider  (1)  how  to  avoid  the  disease  germs  which  are  wide- 
spread, and   (2)  how  to  prevent  the  wide  distribution  of 
disease  germs. 

483.  Avoiding  Disease  Germs.  —  The  methods  of  avoiding 
the  introduction  of  disease  germs  into  one's  body  depend 
upon  the  nature  of  the  disease  and  the  causative  organism. 
As  a  rule,  the  germs  are  introduced  into  the  alimentary  canal 
with  food  and  water,  into  the  respiratory  organs,  into  the 
blood  by  insect  bites,  or  into  wounds. 

484.  Infection  through  Alimentary  Canal.  — Typhoid  fever 
and  Asiatic  cholera  are  good  examples  of  intestinal  diseases 
caused  by  germs  which  are  spread  by  excreta.     Imperfect 
sewerage  and  insects  may  lead  to  contamination  of  various 
foods  (milk,  vegetables,  fruits,  raw  oysters)   and  drinking 
water.     In  places  where  these  diseases  are  common  the  only 
safety  for  the  individual  is  in  the  use  of  cooked  foods  and 
served  hot  or  at  least  guarded  from  contamination  by  flies, 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING       555 

dirty  human  hands,  and  dust.  Salads  and  other  vegetables 
not  cooked  should  at  least  be  carefully  washed  in  water 
which  has  been  boiled. 

If  there  is  reason  to  doubt  the  purity  of  drinking  water, 
heat  it  to  the  boiling  temperature  and  pour  into  sterile 
closed  containers  (such  as  earthen  jugs).  Longer  boiling 
is  unnecessary  and  undesirable.  These  rules  will  protect 
individuals.  How  to  check  the  spread  of  intestinal  diseases 
and  thus  protect  the  public  is  a  problem  of  sewerage,  and  of 
insect  control  (see  §  330) . 

Intestinal  diseases  of  children,  especially  in  warm  weather 
("summer  complaint"),  are  largely  due  to  stale  milk.  If 
very  clean,  fresh  milk  is  not  available,  then  the  milk 
should  be  pasteurized  (§  256).  Thoroughly  wash  and  sterilize 
(§  256)  daily  all  milk-bottles. 

It  should  be  remembered  that  some  bacteria  of  decay 
may  develop  in  foods  and  cause  unhealthful  conditions. 
This  and  the  fact  that  some  disease  germs  will  multiply 
in  milk  and  other  foods  should  lead  to  caution.  Foods 
showing  signs  of  decomposition  should,  of  course,  be  rejected ; 
but  more  important  is  the  protection  of  foods  from  dust, 
insects,  and  growth  of  bacteria.  Remembering  that  bacteria 
do  not  flourish  when  very  cold,  one  thinks  of  the  ice-box  as 
best  for  preventing  growth ;  but  rarely  is  an  ice-box  cold 
enough  to  preserve  highly  decomposable  foods  (such  as 
soup,  gelatin,  milk)  for  many  days.  Best  of  all  methods 
is  heating  to  the  boiling  point  daily  the  foods  to  be  preserved 
temporarily,  and  also  keep  them  in  an  ice-box  from  day  to 
day.  Thus  disease  germs,  if  present,  will  be  killed ;  and 
ordinary  decay,  which  might  produce  ptomaines  (poisons  in 
foods)  is  prevented. 

486.  Infection  through  Respiratory  Organs.  —  Tubercu- 
losis is  a  good  example.  Tubercle  bacilli  may  get  into 
the  lungs  (1)  by  close  contact  of  healthy  with  tuberculous 
persons  (e.g.,  by  kissing,  and  using  common  drinking  cups  or 


556  APPLIED  BIOLOGY 

towels) ;  (2)  from  dust  arising  from  the  dried  sputum  of  a 
tuberculous  victim ;  (3)  possibly  from  milk  of  tuberculous 
cows;  (4)  by  flies  which  carry  the  bacteria  from  sputum. 
The  first  line  of  danger  is  easily  avoided.  Especially  should 
those  suffering  from  tuberculosis  or  whose  friends  are 
afflicted  get  from  the  Society  for  the  Prevention  of  Tuber- 
culosis, in  New  York  City,  the  free  circulars  which  give  rules 
for  preventing  the  spread  of  the  disease,  and  follow  them 
carefully.  Doubtful  milk  should  always  be  pasteurized,  or 
boiled.  The  danger  from  dust  is  impossible  of  avoidance  by 
individuals  so  long  as  the  spitting  nuisance  continues.  For- 
tunately, drying  and  sunlight  kill  most  of  the  germs  out  of 
doors,  and  good  ventilation  and  cleaning  reduce  the  danger 
indoors. 

So  far  as  personal  hygiene  is  concerned,  the  most  important 
preventive  measure  against  pulmonary  tuberculosis  is  keep- 
ing in  general  good  health  by  good  food,  outdoor  exercise, 
fresh  air,  good  sleep,  and  avoiding  colds.  So  long  as  the 
body  is  in  good  condition  there  is  little  danger  of  the  bacteria 
getting  a  chance  to  flourish  in  the  lungs;  and  even  if  the 
disease  has  begun  to  develop,  a  cure  may  be  effected  by 
careful  attention  to  the  rules  of  hygiene  which  physicians 
and  special  books  and  pamphlets  prescribe. 

Many  children's  diseases  (whooping  cough,  measles, 
mumps,  chicken-pox,  etc.)  are  believed  to  be  contracted 
through  the  respiratory  organs.  The  best  protection  is  (1) 
keeping  away  from  cases  of  such  diseases,  and  (2)  avoiding  the 
use  of  pencils,  toys,  towels,  handkerchiefs,  drinking  cups, 
etc.,  which  may  have  been  in  contact  with  the  mouths  of 
other  children,  who  may  carry  the  germs  although  apparently 
well.  These  and  the  biological  rules  given  in  §  488  will  pre- 
vent most  epidemics  of  children's  diseases. 

486.  Infection  by  Insects.  —  The  relations  of  mosquitoes 
and  flies  to  disease  have  been  stated  in  §§  329,  330.  Constant 
warfare  should  be  waged  against  these  insects. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      557 

487.  Infection  in  Wounds.  —  Proper  healing  without  in- 
flammation   depends    upon    keeping    out    bacteria.     Wash 
all  cuts  with  weak  antiseptics  obtained  from  druggists  (e.g., 
carbolic  acid  one  part  to  40  of  water,  or  mercuric  chloride 
tablets  in  water  as  directed  on  bottle.     Listerine  for  slight 
cuts).     Deep   cuts  or  punctures  should   be   dressed   by  a 
surgeon,  for  bacteria  may  have  been  pushed  in  too  deep  to 
be  reached  without  special  instruments,  and  there  is  danger  of 
tetanus  or  septicaemia   (blood  poisoning).     Protect  wounds 
with  sterilized  cotton  and  medicated  gauze.     Adhesive  tape 
should  not  be  put  on  so  closely  as  to  keep  out  the  air ;  and 
collodion  solution  is  of  questionable  value  for  any  except 
shallow  and  well-cleaned  cuts.     Always  consult  a  doctor  at 
once  if  any  marked  inflammation  develops  in  any  wound. 
Delay  in  such  cases  often  proves  serious. 

488.  Preventing  Distribution  of   Disease  Germs.  —  This 
may  be  accomplished  (1)  by  scientifically  dealing  with  cases 
of  germ  diseases,  which  we  discuss  below  under  "  the  bacteriol- 
ogy of  sick-rooms  " ;   and  (2)  by  public  sanitary  control  of 
food-supplies,  water,  sewage,  quarantines,  and  other  matters 
of  public  hygiene. 

489.  Bacteriology  of   Sick-rooms.  —  Since   it  often  hap- 
pens, especially  with  children,  that  germ  diseases  must  be 
treated  at  home,  it  is  important  that  every  citizen  should 
understand  the  scientific  principles  which  doctors  prescribe. 
All  the  rules  depend  upon  the  fact  that  micro-organisms  are 
numerous  in  the  secretions  and  excretions  of  patients,  and 
must  be  destroyed  by  the  methods  of  bacteriology. 

(1)  Isolation.  —  As  soon  as  a  person  becomes  ill,  he  should 
remain  in  one  room  until  the  doctor  is  sure  of  the  diagnosis. 
If  it  proves  to  be  a  contagious  disease,  complete  isolation 
will  be  required  by  the  health  officers.  Even  in  cases  of 
diseases  not  commonly  isolated  it  is  best  that  only  the 
nurse  come  into  contact  with  the  patient,  for  there  is  always 
possible  danger  of  spreading  the  germs  to  healthy  people. 


558  APPLIED  BIOLOGY 

In  the  cases  of  all  germ  diseases,  the  doorway  to  the  patient's 
room  should  be  kept  covered  with  a  damp  sheet  to  prevent  as 
far  as  possible  the  exit  of  dust  particles. 

(2)  Disinfection.  —  Remember  that  all  articles  in  contact 
with  a  patient  may  bear  the  germs.  To  reduce  the  amount 
of  disinfection  which  will  be  necessary,  remove  all  useless 
carpets,  curtains,  and  furniture  as  soon  as  the  disease  is 
diagnosed;  and  thereafter  remove  nothing  from  the  room 
except  under  the  strictest  germicidal  precautions.  Hence 
all  clothing,  bedding,  handkerchiefs,  etc.,  should  be  placed  in 
a  tight  receptacle,  such  as  a  wash-boiler,  covered  with  water, 
and  boiled  for  twenty  minutes  before  they  can  safely  be 
sent  to  a  laundry  or  washed  in  tubs  at  home.  The  wash- 
boiler  should  be  kept  in  the  patient's  room,  and  not  opened 
until  after  boiling.  Its  outside  should  be  wiped  with  a 
cloth  wet  with  strong  washing-powder  solution,  or  other 
germicide,  if  necessary  to  take  it  to  another  room  for 
boiling. 

Spoons,  dishes,  tumblers,  etc.,  should  be  placed  in  a  bucket 
of  water  (preferably  hot  and  containing  much  washing- 
powder)  before  removing  from  the  patient's  room,  and  then 
boiled  for  20  to  30  minutes.  Food  left  by  the  patient  should 
also  be  boiled  or  burned.  Playthings  used  by  children 
should  be  such  as  can  be  washed  in  germicide  solutions, 
boiled,  or  burned  after  convalescence.  A  doll  or  a  "  teddy 
bear,"  if  not  burned,  might  be  a  very  dangerous  carrier 
of  germs.  Books  or  magazines  should  be  of  little  value, 
never  from  libraries,  and  finally  carried  (in  a  closed  bucket) 
to  a  stove  or  furnace. 

The  one  who  attends  the  patient  should  thoroughly  wash 
her  hands  frequently  with  hot  water  and  strong  soap,  or 
germicide  solutions,  and  before  leaving  the  room.  Neglect 
of  this  simple  precaution  has  often  spread  germ  diseases. 
The  habit  of  expert  nurses  of  wearing  washable  dresses 
should  be  imitated  as  far  as  possible. 


BIOLOGY  APPLIED   TO  HEALTHFUL  LIVING      559 

All  excreta  should  be  treated  with  strong  chloride  of  lime 
or  other  disinfectants  sold  by  the  druggists. 

No  sweeping  should  be  allowed  in  a  sick-room.  Dust 
should  be  wiped  up  with  a  damp  cloth  which  should  after- 
ward be  washed  out  in  boiling  water  with  washing-powder. 

After  recovery  of  the  patient,  fumigate  (disinfect  by  fumes 
or  gas)  the  room  with  formaldehyde  (§  256, /),  leaving  in  the 
room  all  furniture,  pictures,  matresses,  books,  etc.,  and  spread 
the  articles,  open  the  drawers,  wardrobe,  etc.,  so  as  to  allow 
free  circulation  of  the  gas.  Do  not  remove  rugs,  curtains, 
clothing  or  any  'article  from  the  room  until  after  thorough 
disinfection. 

These  precautions  may  seem  extremely  detailed,  but 
bacteriologists  have  shown  that  they  are  necessary  in  order 
to  safeguard  against  distribution  of  dangerous  germs.  It  is 
the  duty  of  every  citizen  to  aid  in  popularizing  and  putting 
into  practice  knowledge  by  which  some  of  the  most  dangerous 
diseases  may  be  made  extremely  rare  in  occurrence. 

490.  Public  Hygiene.  —  Since  a  large  number  of  people 
are  ignorant  of  scientific  principles  or  have  no  regard  for 
the  health  of  other  people,  it  has  become  necessary  to  institute 
public  sanitary  control  of  foods,  water,  sewage,  quarantines, 
etc.,  in  all  cities  and  to  a  certain  extent  in  the  country  at 
large.  Thus  the  official  representatives  of  the  government  are 
charged  with  the  duty  of  protecting  citizens  against  disease 
in  cases  where  the  individuals  cannot  exert  control.  For 
example,  a  city  board  of  health  can  force  dealers  to  sell  only 
clean  milk,  but  without  such  public  control  the  individual 
citizen  must  accept  the  impure  milk  which  the  dealer 
offers  for  sale.  Likewise,  under  public  sanitary  control, 
meats  and  other  foods  can  be  inspected,  and  made  to  meet 
the  requirements  of  the  law ;  water  can  be  obtained  from  the 
purest  possible  sources;  sewage  systems  can  be  arranged 
to  avoid  danger  from  disease  germs;  streets  and  other 
public  places  kept  clean;  quarantine  of  dangerous  diseases 


560  APPLIED  BIOLOGY 

established;  vaccination  required;  spitting  on  walks  pro- 
hibited; and  still  other  measures  in  the  interest  of  public 
health  put  into  legal  operation.  Such  is  the  field  of  public 
hygiene  and  the  work  of  health  officials,  who  aim  to  apply 
biological  principles  and  Protect  the  citizens  in  numerous 
cases  where  individuals  c.f  mot  protect  themselves.  Every 
citizen  should  get  acquainted  with  the  health  laws  in  his 
locality  and  then  cooperate  with  the  officials  whose  duty 
it  is  to  get  the  laws  enforced. 


PART   V 


CHAPTER  XIX 

EVOLUTION   AND    HEREDITY    OF   ANIMALS   AND 
PLANTS 

491.  Definition  of  Evolution.  —  Many  times  in  the  pre- 
ceding chapters  we  have  had  occasion  to  point  out  that 
similarity  of  structures  suggests  relationship  of  animals  and 
plants,  and  that  all  organisms  have  undergone  more  or  less 
change  in  adaptation  to  the  environments  in  which  they  live, 
A  remarkable  set  of  examples  are  the  adaptations  of  roots, 
stems,  leaves,  flowers,  and  fruits  described  in  Chapter  VIII. 
In  all  these  cases  the  similarity  of  structure  suggests  that 
the  adapted  organs  (roots,  stems,  etc.)  have  been  derived 
from  the  typical  forms  of  these  organs  as  seen  in  other  plants. 
For  example,  fleshy  roots  have  developed  from  plants  with 
ordinary  tap-roots,  irregular  flowers  adapted  to  insects 
probably  descended  from  regular  flowers;  and  so  in  all 
special  adaptations  of  plants  close  examination  suggests 
the  line  of  modificat  on  in  making  special  adaptations  and 
points  out  the  relationship. 

As  an  example  on  the  animal  side,  it  has  been  shown  that 
the  various  forms  of  feet  of  hoofed  mammals  appear  to  have 
been  derived  from  ancestors  with  five  toes  (§  361),  and  that 
general  similarity  in  structure  of  all  parts  of  their  bodies  points 
to  relationship  between  all  the  ungulates.  Such  facts  of 
similarity  which  suggest  relationship  are  fundamental  to 
the  study  of  Organic  Evolution,  by  which  is  meant  the  theory 
2o  561 


562  APPLIED  BIOLOGY 

that  all  organisms  which  are  now  on  the  earth  have  been 
derived  from  and  consequently  are  directly  related  to  organisms 
which  previously  existed  in  the  past  history  of  the  earth ; 
and  that  similarities  or  homologies  of  structure  are  due  to 
common  descent  from  earlier  forms.  To  give  a  concrete 
illustration :  the  similarities  of  existing  vertebrates  are  be- 
lieved to  mean  that  they  have  descended  from  an  ancient 
type  of  animal  which  had  the  general  plan  of  body  (§  344) 
now  common  to  all  existing  vertebrates ;  the  similarity  of 
existing  birds  that  they  are  the  direct  descendants  of  an  an- 
cient type;  and  so  on  for  all  groups  of  animals.  Hence,  all 
vertebrates  are  more  or  less  closely  related  to  each  other,  the 
degree  of  relationship  being  indicated  by  the  closeness  of 
similarity  (e.g.,  a  fish  and  a  mammal  distantly  related ;  a  dog 
and  a  wolf  closely). 

492.  History  of  Evolution.  —  This  idea  that  animals  have 
been  derived  from  other  animals  is  a  very  old  one  dating  back 
to  some  philosophers  among  the  ancient  Greeks,  but  it  was  not 
developed  in  a  thoroughly  scientific  form  before  the  nineteenth 
century.  In  1859  Charles  Darwin,  of  England,  published 
a  book  entitled  "  The  Origin  of  Species,"  in  which  he  mar- 
shalled such  a  convincing  array  of  facts  to  prove  the  evolu- 
tion of  animals  and  plants  that  the  theory  was  soon  accepted 
by  most  scientific  men.  Following  Darwin's  suggestions, 
hundreds  of  scientific  men  began  to  investigate  all  the  facts 
which  seemed  to  be  connected  with  evolution,  and  the  result 
is  that  the  theory  has  been  universally  accepted  in  the  scien- 
tific world  as  the  only  explanation  for  the  remarkable  simi- 
larity of  organisms  which  has  so  often  attracted  our  attention 
in  previous  chapters.  To-day  there  is  no  famous  living 
botanist  or  zoologist  who  does  not  believe  in  the  theory  of 
evolution;  that  is,  that  species  of  animals  and  plants  have 
originated  by  descent  with  modification  of  pre-existing  forms. 
The  entire  scheme  of  classification  of  animals  and  plants 
which  is  now  in  use  by  all  biologists  is  based  on  the  idea  of 


EVOLUTION  OF  ANIMALS  AND  PLANTS          563 

relationship  through  descent.  Thus  we  classify  together  the 
fishes,  amphibians,  reptiles,  birds,  and  mammals  because  the 
general  plan  of  structure  is  so  similar  in  all  these  vertebrates 
as  to  suggest  that  they  have  descended  from  some  common 
ancestors  or  primitive  vertebrates  which  lived  in  the  far- 
distant  ages  with  which  geology  deals. 

493.  Evidences  of  Evolution.  —  In  this  book  we  can  con- 
sider only  briefly  the  lines  of  evidence  which  led  Darwin  and 
his  followers  to  accept  the  idea  of  evolution  as  a  true  state- 
ment of  the  facts  observed  in  study  of  animals  and  plants. 
These  evidences  will  be  outlined  under  the  following  para- 
graphs :    Structural  Evidences ;    Embryological  Evidences ; 
Geological   Evidences;    Distribution  Evidences;    and  Ex- 
perimental Evidences. 

494.  Structural  Evidences  of  Evolution.  —  In  almost  every 
lesson  of  this  book  we  have  had  occasion  to  refer  to  similarity 
of  structures  in  organisms.     Such  similarity  is  the  most  re- 
markable fact  in  biology ;  and  the  only  scientific  explanation 
is  that  organisms  have  been  evolved  or  have  developed  from 
common  ancestors. 

Especially  striking  are  the  facts  in  the  case  of  many  ad- 
aptations. Whales  have  become  adapted  to  aquatic  life, 
but  the  lost  hind-legs  are  still  represented  by  rudimentary 
bones  far  beneath  the  skin.  Snakes  appear  to  have  been 
derived  from  ancestors  with  legs,  but  only  a  few  species 
(python,  boa)  show  the  rudiments  of  the  bones  of  the  hind- 
legs.  Wingless  birds  (ostrich,  Apteryx)  have  small  rudiments 
of  useless  wings.  Horses  still  show  the  rudimentary  bones  of 
two  lost  toes.  And  in  special  books  one  can  find  records  of 
thousands  of  such  cases  of  rudimentary  structures  which  point 
to  relationship  with  forms  in  which  the  organs  are  fully  de- 
veloped and  functional.  In  short,  every  known  adaptation 
of  animals  and  plants  gives  evidence  of  evolution  having 
occurred,  for  the  adapted  structures  obviously  have  originated 
by  change  and  modification  of  previously  existing  structures. 


564  APPLIED  BIOLOGY 

We  cannot  explain  the  presence  of  a  rudimentary  organ 
(such  as  an  Apteryx  wing,  or  whale  hind  limb)  except  as  an 
adaptation  or  change  of  a  corresponding  organ  in  closely 
similar  animals.  Hence  the  Apteryx  must  have  descended 
from  birds  with  wings,  and  whales  from  mammals  with  hind 
legs.  Such  is  the  line  of  interpretation  which  biologists  now 
apply  everywhere  in  the  animal  and  plant  kingdoms. 

495.  Embryological  Evidences  of  Evolution.  —  In  §  364 
we  have  noted  that  gill-slits  appear  in  the  embryos  of  all 
vertebrates,  although  only  fishes  and  young  amphibians  make 
use  of  them.  The  appearance  of  embryonic  organs  which 
never  develop  is  very  common  in  every  group  of  animals,  and 
also  in  plants.  The  case  of  gill-slits  is  only  one  of  many  such 
cases  among  higher  vertebrates. 

For  such  appearance  of  useless  embryonic  organs  there  is 
only  one  satisfactory  explanation;  namely,  that  they  are 
evidences  of  ancestral  history.  Zoologists  now  agree  in  believ- 
ing that  the  presence  of  gill-slits  in  embryos  of  all  verte- 
brates means  that  all  vertebrates  were  derived  from  fish- 
like  ancestors  which  needed  gill-slits  for  respiration.  But 
why  gill-slits  should  still  persist  in  the  embryology  of  every 
vertebrate  is  a  mystery.  We  can  simply  point  to  the  facts 
indicating  that  it  is  inherited  from  an  ancestral  condition, 
an  heirloom  from  the  long-past  history  of  the  race  of  back- 
boned animals. 

In  still  another  way  does  the  embryology  of  animals  sug- 
gest evolution  from  common  ancestors,  and  that  is  in  the 
similar  development.  In  formation  of  egg-cells  and  sperm- 
cells,  in  fertilization  of  eggs,  in  cell-division,  and  in  formation 
of  organs  there  is  a  surprising  similarity  among  animals ;  and 
m  case  of  animals  whose  structure  shows  close  relationship 
the  similarity  extends  into  great  detail.  Such  facts  lead  to 
only  one  interpretation;  namely,  that  all  animals  have  been 
leveloped  from  the  first  forms  of  life.  The  facts  from  embry- 
ology are  so  convincing  as  to  relationship  that  classification 


EVOLUTION  OF  ANIMALS  AND  PLANTS          565 

of  an  animal  or  plant  is  not  considered  absolutely  decided  un- 
til its  embryology  has  been  investigated  and  compared  with 
that  of  its  allies.  Animals  and  plants  which  are  often  highly 
modified  when  adult  are  usually  much  like  their  near  rela- 
tives when  in  embryonic  stages.  By  this  means  it  has  been 
determined  that  some  wingless  insects  are  related  to  others 
which  have  wings ;  that  the  sac-like  barnacle  (§  316)  and  the 
goose-barnacle  are  crustaceans;  and  so  in  hundreds  of  cases 
study  of  embryos  of  animals  and  plants  have  disclosed  hidden 
resemblances  'which  mean  relationship. 

496.  Geological  Evidences  of  Evolution.  —  If  present  ani- 
mals and  plants  have  descended  from  ancient  ones,  we  ought 
to  find  some  evidence  of  changes  in  the  fossils.     This  is 
exactly  what  has  been  learned  from  study  of  the  past  history 
of  organisms  preserved  in  the  rocks.     A  vast  number  of 
species  of  many  groups  of  animals  and  plants  have  now  been 
collected  and  studied,  and  all  the  facts  learned  make  biologists 
more  than  ever  convinced  that  the  theory  of  evolution  or 
descent  is  true.     One  illustration  must  suffice  for  our  present 
study,  and  that  is  the  fossils  of  ancestral  horses  showing 
gradual  reduction  of  toes  to  one  on  each  foot  which  have  been 
found  (§§  361,  363).     In  the  case  of  thousands  of  other  species 
of  organisms,  the  great  museums  now  contain  fossils  which 
help  in  the  study  of  relationships  of  both  past  and  present 
organisms.     Popular  books  in  this  line  are  Lucas's  "  Ani- 
mals of  the  Past  "  or  Lancaster's  "  Extinct  Animals."     Simi- 
lar popular  books  on  plant  fossils  have   not  been  written 
because  there  is  not  such  widespread  knowledge  and  interest 
in  ancient  plants  as  in  the  animals.     However,  the  fossil 
plants  are  to  biologists  no  less  interesting  and  convincing 
than  the  animals. 

497.  Distribution   Evidences   of  Evolution.  —  Briefly  we 
may  state  the  central  fact  in  this  line  as  follows :  Organisms 
which  appear  in  their  similar  structure  and  embryonic  de- 
velopment to  be  clearly  related  are  commonly  found  in  regions 


566  APPLIED  BIOLOGY 

that  were  easily  accessible  to  the  descendants  of  their  com- 
mon ancestors.  The  animals  and  plants  on  islands  near  con- 
tinents are  very  like  those  of  the  mainland.  Within  any 
continent,  the  species  show  a  correlation  with  the  possibility 
of  distribution.  For  example,  the  crayfishes  of  eastern  North 
America,  though  of  many  species,  belong  to  one  genus.  On 
the  west  coast  there  is  another  genus.  As  might  be  expected, 
the  high  mountains  have  kept  the  two  genera  apart.  There 
are  thousands  of  known  cases  in  which  mountains,  rivers,  and 
seas  have  been  barriers  confining  groups  of  animals  or  plants 
to  a  given  territory,  with  the  result  that  their  descendants 
show  close  relationship. 

The  fact  that  there  are  many  species  of  both  animals  and 
plants  which  occur  in  both  Asia  and  America  and  in  Europe 
and  America  suggests  that  there  was  once  some  land  con- 
nection over  which  the  ancestors  of  existing  forms  might  have 
passed  to  America.  That  such  a  connection  once  existed 
in  the  region  of  Behring  Sea  seems  probable  to  geologists. 

Another  interesting  series  of  facts  of  distribution  which 
point  to  evolution  are  those  relating  to  introduction  of  new 
species  into  regions  where  they  do  not  live  naturally.  In 
America  we  have  numerous  weeds,  insects,  English  sparrows 
and  other  Old  World  organisms  which  are  certainly  well 
adapted  to  the  conditions  here.  Why  then  were  they  not  here 
naturally?  The  answer  is  that  they  were  developed  in 
limited  geographical  territory  where  their  direct  ancestors 
and  near  allies  lived.  This  is  why  hundreds  of  species  of 
organisms  occur  only  in  one  part  of  the  world,  often  on  a  single 
island,  in  a  valley,  river,  or  other  region  isolated  by  natural 
barriers. 

498.  Experimental  Evidences.  —  The  most  convincing 
evidence  of  descent  is  obtained  from  study  of  domesticated 
animals  and  plants,  with  which  for  thousands  of  years  man 
has,  consciously  and  unconsciously,  been  experimenting. 
That  organisms  are  changeable  is  proved  beyond  doubt  by 


EVOLUTION  OF  ANIMALS  AND  PLANTS          567 

the  hundreds  of  varieties  of  animals  and  plants  which  are 
raised  under  domestication.  For  example,  we  have  dozens 
of  varieties  of  chickens  at  the  poultry  shows  and  most  of  them 
are  known  to  have  descended  from  certain  breeds ;  and  prob- 
ably all  of  them  came  from  one  or  few  wild  species,  one  of 
which  was  certainly  the  East  Indian  jungle-fowl.  Several 
hundred  varieties  of  pigeons  have  developed  from  one  (or 
possibly  a  few)  species  of  wild  pigeon.  Numerous  varieties 
of  many  common  plants  cultivated  for  food,  ornament,  or 
other  use  have  been  selected  and  improved  by  horticulturists. 
In  short,  any  one  who  looks  through  books  describing  the 
numerous  breeds  or  varieties  of  common  domesticated  ani- 
mals and  plants  will  not  doubt  the  fact  that  organisms  do 
change.  And  it  must  be  remembered  that  most  of  these 
changes  under  domestication  have  occurred  within  a  few 
hundred  years ;  while  tens  of  thousands  of  years  have  been 
available  for  changes  in  wild  animals  and  plants. 

It  should  be  noted  that  man  has  affected  animals  and  plants 
under  his  care  chiefly  by  selecting  peculiar  individuals. 
When  we  speak  of  making  a  new  variety  of  corn  or  a  new 
breed  of  cattle  we  simply  mean  that  we  have  selected  for 
propagation  certain  individuals  that  by  nature  were  better 
than  their  relatives.  For  example,  breeds  of  hornless  cattle 
have  within  recent  years  been  started  by  selecting  individuals 
born  without  power  of  growing  horns  and  able  to  transmit 
this  characteristic  to  their  progeny.  Likewise,  horticulturists 
select  every  year  certain  extra-valuable  plants  and  from  them 
obtain  seed  of  new  varieties.  With  new  varieties  of  either 
animals  or  plants  improvements  may  be  made  with  each  new 
generation  by  selecting  the  best  individuals  as  parents.  For 
example,  the  hornless  cattle  are  now  being  improved  rapidly 
by  selecting  for  propagation  only  those  which  are  entirely 
hornless  and  which  are  also  excellent  for  milk  or  meat.  This 
is  an  illustration  of  the  kind  of  selection  which  scientific 
agriculturists  are  constantly  applying  to  all  kinds  of  animals 


568  APPLIED  BIOLOGY 

and  plants  under  domestication.  It  is  often  called  artificial 
selection  to  distinguish  from  natural  selection  (§  500). 

499.  Struggle  for  Existence.  —  Long  before  Darwin  wrote 
the  "  Origin  of  Species  "  (1859)  it  was  recognized  by  scien- 
tific men  that  vastly  more  individual  animals  and  plants  came 
into  the  world  than  can  possibly  survive,  for  there  is  not  room 
and  food  for  them  all.  To  illustrate  :  Probably  not  one  oyster 
embryo  in  a  million  grows  to  maturity;  and  if  none  of  them 
perished  for  a  very  few  generations,  the  oceans  would  be 
solid  with  oysters. 

Elephants  multiply  slower  than  all  other  animals,  but  if 
all  elephants  lived  one  hundred  years  and  produced  but  six 
young  per  pair,  there  would  be  after  800  years  about  19,000,- 
000  living  descendants  of  one  pair.  Imagine  each  of  these 
descendants  (9,500,000  pairs)  reproducing  at  the  same  rate 
for  another  800  years  !  And  yet  elephants  have  been  on  this 
earth  many  times  800  years  and  living  elephants  are  not  very 
numerous  anywhere.  Evidently  the  vast  majority  do  not 
live  long  enough  to  produce  six  young  per  pair.  In  fact, 
most  of  them  do  not  live  through  the  30  years  required  to 
reach  maturity. 

Among  plants,  we  have  only  to  watch  a  patch  of  weeds 
crowding  each  other  and  the  cultivated  plants  in  a  garden 
in  order  to  see  how  severe  is  the  struggle  resulting  from  over- 
population. Suppose  an  annual  plant  this  year  produces 
100  seeds,  that  each  of  these  next  year  will  form  a  plant  with 
100  seeds,  and  so  on  for  many  years.  There  would  be  100 
plants  next  year,  10,000  the  second  year,  1,000,000  the  third 
year,  and  so  on  in  geometrical  ratio  multiplied  by  100  each 
year.  Evidently  the  world  would  soon  be  full  of  that  kind 
of  plant.  But  although  many  plants  produce  far  more  than 
100  seeds  annually,  none  of  them  increase  at  such  a  rate. 
Obviously,  the  vast  majority  of  seeds  do  not  develop  into 
mature  plants.  Only  a  very  small  proportion  of  the  sum 
total  of  all  kinds  of  seeds  formed  in  any  one  year  could  ever 


EVOLUTION   OF  ANIMALS  AND  PLANTS          569 

develop  beyond  the  seedling  stage,  for  there  is  not  space  and 
food  enough.  The  vast  majority  will  be  eaten  by  animals,  de- 
stroyed by  unfavorable  weather,  or  killed  by  crowding  such 
as  one  can  see  in  any  place  where  plant  seedlings  are  numerous. 

Turn  where  we  will  among  animals  and  plants,  the  facts 
are  essentially  as  stated  for  the  oyster  and  weeds :  the  total 
number  of  young  organisms  is  so  enormous  that  there  must 
be  a  struggle  for  existence  and  destruction  of  the  vast  majority. 
This  leads  to  many  complicated  relations  between  organisms. 
Food  must  be'  obtained,  and  this  means  competition  between 
those  which  require  the  same  kind  of  food,  between  carnivora 
and  their  prey,  and  between  animals  and  plants.  Also,  there 
is  a  constant  struggle  with  the  environment  (e.g.,  crowded 
plants  struggling  for  light  and  water;  and  various  animals 
against  adverse  climatic  conditions).  These  are  only  sug- 
gestions of  some  ways  in  which  nature  subjects  organisms 
to  struggle  in  an  intense  competition,  which  is  caused  chiefly 
by  the  fact  that  all  forms  of  life  reproduce  more  rapidly  than 
necessary  to  maintain  a  constant  number  of  individuals  of 
each  species. 

There  are  reasons  for  believing  that  the  struggle  for  exist- 
ence does  not  result  in  indiscriminate  destruction  of  indi- 
viduals. Some  are  better  fitted  than  others  to  withstand  the 
adverse  conditions.  For  example,  plant  twenty  sunflower 
seeds  in  a  flower-pot,  and  gradually  one  or  two  of  the  seed- 
lings will  grow  above  the  others.  They  are  best  fitted  and 
therefore  win  in  the  struggle  for  existence.  The  same  thing 
is  constantly  happening  in  all  species  of  organisms.  Those 
which  survive  and  reproduce  are  the  fittest.  The  weakest 
and  least  adapted  soon  perish.  Thousands  of  forms  of 
animals  and  plants  which  once  lived  on  the  earth  have  dis- 
appeared because  they  were  not  properly  fitted  for  the  struggle 
with  changed  environment  and  improved  competitors.  The 
effect  of  all  this  upon  origin  of  new  species  will  be  discussed 
in  the  next  paragraph. 


570  APPLIED  BIOLOGY 

500.  Natural  selection,  meaning  selection  by  nature,  is  a 
term  applied  to  the  preservation  of  the  favored  or  best 
adapted  individuals  in  the  struggle  for  existence  (§  499). 
For  example,  a  grasshopper  with  legs  adapted  for  jumping 
farther  than  can  other  individuals  of  the  same  kind  has  a 
better  chance  of  surviving  by  escaping  enemies.  Hence,  it 
is  said  that  nature  selects  such  best  fitted  individuals. 
These  will  have  a  chance  to  propagate ;  and,  according  to  the 
laws  of  heredity  (§  501),  will  tend  to  transmit  their  own  pecul- 
iar structure  to  descendants. 

Darwin's  "  Origin  of  Species,"  was  designed  to  show  how 
natural  selection  working  for  tens  of  thousands  of  years  and 
generations  might  have  led  to  great  changes  in  organisms. 
For  example,  the  best  jumping  grasshoppers  surviving  in 
each  generation  would  lead,  in  the  course  of  long  series  of 
years,  to  the  perfectly  adapted  legs  of  existing  grasshoppers. 
Thus  a  new  kind  or  species  might  have  arisen. 

Only  one  organ  (leg)  has  been  considered  above,  but  it 
should  be  noted  that  any  important  organ  might  affect  the 
question  of  survival  in  the  same  way. 

It  also  should  be  noted  that  only  useful  things  could  be 
selected  by  nature.  For  example,  a  tumbler-pigeon,  which 
occasionally  turns  somersaults  while  flying,  would  not  have 
been  preserved  by  natural  selection.  In  fact,  the  habit  is 
not  only  useless,  but  in  nature  probably  would  lead  to  de- 
struction by  birds  of  prey  which  could  easily  capture  a  pigeon 
during  the  delay  in  tumbling.  But  under  human  care  and 
artificial  selection,  these  peculiar  pigeons  have  been  pre- 
served as  curiosities  or  freaks,  and  allowed  to  multiply. 

Numerous  peculiarities  of  structure  and  color  in  many 
domesticated  animals  and  plants  have  been  preserved  by 
artificial  selection;  but  not  being  useful  would  not  have 
developed  under  natural  selection. 

Seedless  plants  and  double  flowers  tending  to  become 
seedless  could  not  have  developed  except  under  artificial 


EVOLUTION  OF  ANIMALS  AND  PLANTS  571 

selection.  The  fact  that  natural  selection  acts  only  by  sur- 
vival of  the  fittest  in  the  struggle  for  existence,  limits  its 
effect  to  things  which  make  animals  and  plants  better 
adapted  to  their  environment.  On  the  other  hand,  man  can 
control  conditions  so  as  to  select  for  preservation  under 
domestication  any  peculiar  individuals  which  interest  him. 
This  is  why  we  have  such  vast  numbers  of  varieties  of  cul- 
tivated plants  and  domesticated  animals. 

501.  Heredity.  —  It  is  a  well-known  fact  that  offspring 
tend  to  resemble  their  parents  closely.  We  commonly  speak 
of  characteristics  which  previously  appeared  in  the  parents 
or  even  earlier  progenitors  as  inherited  or  due  to  heredity. 

What  may  be  Inherited.  —  We  have  previously  noted  that 
"  like  tends  to  produce  like,"  which  means  that  each  animal 
and  plant  has  the  power  of  transmitting  its  own  general 
characteristics  to  its  offspring,  e.g.,  a  frog  transmits  frog 
characteristics,  a  bird  those  of  its  own  species,  and  so  on. 
Such  characteristics  which  are  part  of  the  constitution  of 
organisms  are  often  called  germinal,  which  means,  in  the  germ 
(i.e.,  ova  and  sperm  cells). 

Characteristics  developed  during  the  lifetime  of  any  indi- 
vidual, dating  from  the  fertilized  egg,  are  said  to  be  acquired. 
Any  change  in  structure  due  to  use  or  accident,  e.g.,  develop- 
ment of  muscles  by  exercise,  or  loss  of  organs  by  accidents  or 
surgical  operations,  produces  an  acquired  characteristic. 

Stating  briefly  the  essential  facts  of  heredity  as  now  known, 
characteristics  acquired  during  the  lifetime  of  individuals  are 
not  transmitted  in  heredity,  while  germinal  ones  are  capable 
of  inheritance.  For  example,  the  horns  have  been  removed 
from  many  cattle,  the  appendix  from  many  human  individuals, 
tails  from  sheep  and  certain  breeds  of  dogs ;  and  in  no  case 
has  the  removal  of  any  organ  from  a  parent  caused  the  birth 
of  offspring  without  the  organ.  In  short,  such  acquired 
characteristics  are  not  inherited.  On  the  other  hand,  a  cow 
which  was  germinally  hornless  (i.e.,  born  without  ability  to 


572  APPLIED  BIOLOGY 

grow  horns)  would  probably  transmit  the  tendency  towards 
absence  of  horns  to  a  considerable  percentage  of  her  off- 
spring, and  these,  in  turn,  would  tend  to  produce  hornless 
offspring.  In  fact,  the  breeds  of  hornless  cattle,  which  are 
now  becoming  popular  among  farmers,  have  been  developed 
by  selecting  for  breeding  certain  individuals  born  without 
the  beginnings  of  horns.  Likewise,  dogs  and  cats  born  with 
short  tails  are  likely  to  transmit  that  germinal  characteristic, 
which  in  some  unknown  way  is  carried  in  the  reproductive 
cells  from  parent  to  offspring. 

The  above  paragraph  states  the  facts  verified  by  careful 
observation  in  hundreds  of  cases.  In  fact,  the  numerous 
varieties  of  domesticated  animals  and  plants  have  originated 
by  man's  selection  of  individuals  which  during  their  em- 
bryonic history  began  to  develop  peculiarities.  If  these 
peculiarities  make  the  individual  decidedly  different  from 
its  parents,  a  new  breed  might  be  originated,  as  from  the 
first  hornless  cow  from  horned  ancestors.  Usually,  however, 
the  peculiarities  are  little  things  which,  if  selected  by  man, 
will  make  an  improvement  in  the  breed.  Hence  the  scientific 
farmer  is  continually  watching  for  young  animals  which  show 
some  slight  improvement  over  their  parents;  and  he  goes 
through  his  fields  in  search  of  corn  and  other  plants  which 
are  better  than  the  others.  This,  in  brief,  is  the  secret  of  the 
remarkable  improvement  in  almost  all  kinds  of  farm  animals 
and  cultivated  plants  in  the  past  fifty  or  one  hundred  years. 

502.  Heredity  and  the  Germ-cells.  —  Since  an  offspring 
resembles  each  of  its  parents  (a  fact  especially  striking  when 
the  two  parents  belong  to  different  varieties  and  the  off- 
spring is  a  hybrid) ,  it  is  evident  that  the  egg-cell  and 
the  sperm-cell  which  united  to  produce  the  new  individual 
were  the  bearers  of  heredity.  In  fact,  it  has  been  shown 
that  the  chromosomes  in  the  nuclei  of  the  egg-cell  and  sperm- 
cell  contain  the  hereditary  substance,  the  nature  of  which  is 
unknown.  Since  each  germ-cell  contributes  one-half  of  the 


EVOLUTION  OF  ANIMALS  AND  PLANTS          573 

• 

chromatin  of  the  fertilized  egg-cell  in  all  animals  and  plants, 
it  follows  that  the  new  individual  inherits  from  the  two 
parents.  However,  there  may  be  a  more  striking  resem- 
blance to  one  parent.  For  example,  the  offspring  of  a  pair 
of  pigs  of  which  one  is  white  and  one  black  are  often  some 
white,  some  black,  and  some  spotted  black  and  white.  But 
the  white  and  the  black  ones  have  also  inherited  from  each 
parent,  although  in  color  they  show  relationship  to  only  one 
parent.  It  often  happens  that  a  pig  which  in  color  resembles 
one  parent  will,  in  shape  of  head  and  body,  or  other  char- 
acteristics resemble  the  other. 

503.  Law  and  Order  in  Biology.  —  One  who  has  never 
studied  biology  might  look  upon  a  vast  museum  of  natu- 
ral history  as  a  chaotic  mass  of  specimens;  but  biologic 
science  has  reduced  them  to  order.  There  are  many  hun- 
dred thousand  kinds  or  species  of  living  things  wiiich  can 
be  distinguished  from  one  another;  but  after  all,  they  are 
remarkably  similar,  for  they  are  dependent  upon  the  same 
fundamental  substance,  protoplasm,  which  must  perforce 
carry  on  the  same  essential  life-processes  in  all  plants  and 
animals. 

And  what  we  find  in  biology  is  true  in  every  other  natural 
science.  Everywhere  in  nature  there  is  law  and  order. 
Planets  and  comets  move  in  definite  orbits,  light  and  heat 
and  electricity  are  subject  to  unchanging  laws,  elements 
unite  and  separate  according  to  fixed  principles  —  in  short, 
all  things  in  nature  are  conducted  in  accordance  with  law. 

It  has  been  one  aim  of  this  book  to  make  the  reader  realize 
that  the  whole  organic  world,  the  field  of  biology,  is  subject 
to  definite  laws  to  which  the  human  species  is  by  no 
means  an  exception.  Man  is  certainly  an  integral  part  of 
organic  nature,  and  it  behooves  him  to  study  and  apply 
the  discovered  laws  of  biology  upon  which  the  continued 
advancement  of  the  human  race  will,  in  no  small  measure, 
depend. 


INDEX 


[The  figures'given  refer  to  pages,  not  to  sections.] 


Absorption,  50,  473,  479;    by  roots, 

88,  91. 

Accommodation,  517. 
Adaptation,  ~28 ;   birds,  429  ;   flowers, 

199-202;      insects,     398;      leaves, 

191 ;    mammals,  437  ;    roots,  159  ; 

seed-plants,  228;    stems,  180,  184. 
Adenoids,  503. 
^Esophagus,  see  esophagus. 
Age,  of  animals,  17 ;   plants,  20. 
Air,  breathed,  505.    See  oxygen. 
Akene,  218. 
Albuminoids,  463. 
Alcohol,  269,  273,  275,  531,    539-553. 

See  also  yeast,  and  fermentation. 
Algae,  245-252. 
Amoeba,  307-311. 
Amphibia,  65 ;  424-426. 
Anaerobes,  286. 
Anatomy,     definition    of,     21.      See 

structure.  . 
Annuals,  177. 
Angiosperms,  213. 
Annelids,  347-353. 
Anterior,  26. 
Antitoxins,  293. 
Ants,  raising  mushrooms,  266. 
Appendages,  of  crayfish,  361. 
Appendix,  human,  470. 
Aptera,  387. 

Aquarium,  balanced,  128. 
Arachnids,  376-380. 
Arteries,  31 ;   control  of,  491. 
Arthropods,  358-403. 
Assimilation,  15,  109,  120,  304.    See 

also  nutrition,  and  metabolism. 


Bacteria,  276-298 ;   in  hygiene,  554- 

560. 
Bacteriology,  and  health,  554-560. 


Barnacle,  374-376. 
Bathing,  533,  534. 

Bean  plant,  classification,  121 ;  physi- 
ology, 85-120 ;    reproduction,  76- 

85 ;  structure,  67-85. 
Bees,  386. 
Biennial,  156,  177. 
Biogenesis,  346. 
Biology,  definition  of,  2 ;   applied,  3  ; 

animal,     23-65;      plant,     66-121; 

human,  457-560. 
Birds,  428-436. 
Bladder-worm,  342. 
Blights,  260. 
Blood,  uses  of,  50,  53-55 ;    animals 

with,  353 ;    human,  482.    See  also 

circulation. 
Blood-cells,  312,  483. 
Body-cavity,  330 ;   frog,  29 ;   human, 

458 ;  worm,  350. 
Body-wall,  human,  458. 
Bone,  42,  417,  457. 
Botany,  definition  of,  2. 
Brain,  human,  513. 
Branching,  of  stems,  165. 
Bread  making,  273,  274. 
Breathing,     animal,     15 ;      chemical 

tests    for,    16,  19;    human,    504; 

hygiene,  526-528;   plant,  19.     See 

also  respiration. 
Bronchi,  503. 
Bryophytes,  245. 
Budding,     175,    227;      corals,    337; 

hydra,  328 ;  hydroids,  331 ;  worm, 

349  (Fig.  112). 
Buds,  71,  164.    See  also  budding  and 

reproduction. 
Bulblets,  226. 
Bulbs,  183. 
Butterfly,  382. 

C 


Cactus,  stem,  183. 
Calorie,  495. 


575 


576 


INDEX 


Calorimeter,  495,  496. 

Cambium,  71,  174. 

Capillaries,  32,  485. 

Carbohydrates,  100-106;   460-462. 

Carbon,  12  ;   cycle  of,  126. 

Carbon  dioxide,  in  plants,  113;  in 
animals,  54  ;  in  human,  507.  See 
also  excretion. 

Castor-oil  seed,  148. 

Caulicle,  83. 

Cecropia,  383. 

Cell-body,  41. 

Cell-division,  59,  198. 

Cells,  39-44,  69,  71,  108-110,  122; 
use  of  foods,  492;  relation  to 
nutritive  processes,  509. 

Cell-wall,  41. 

Centipede,  379. 

Cephalopods,  414. 

Cephalothorax,  359. 

Cerebellum,  513. 

Change,  physical,  5;  chemical,  6; 
of  matter,  5  ff. 

Characteristics,  139. 

Chemistry,  6. 

Chlorophyll,  75 ;  and  plant  food,  99- 
102 ;  in  Hydra,  325. 

Chloroplast,  75. 

Chordata,  417. 

Chromosomes,  59. 

Cicada,  382,  385. 

Cilia,  301 ;  Fig.  16. 

Circulation,  animal,  55  ;  plant,  130 ; 
simplest  animals,  305 ;  crayfish, 
366;  earthworm,  351;  Hydra, 
327;  human,  486-492.  See  also 
under  blood,  and  lymph. 

Clam,  405. 

Class,  definition,  136. 

Classification,  principles  and  tables, 
133-144;  frog,  64;  bean,  121; 
ferns,  240;  mosses,  245;  spore- 
plants,  233;  protozoans,  319; 
coelenterates,  339;  "worms,"  354; 
insects,  387-390 ;  echinoderms,  357 ; 
mollusks,  416;  vertebrates,  417; 
fishes,  422;  amphibians,  426; 
reptiles,  428;  birds,  433;  mam- 
mals, 436 ;  man,  455. 

Cloaca,  33. 


Coagulation  of  blood,  485. 

Cocoon,  of  worm,  352;    insect,  383, 

384. 

Cod  fish,  422. 
Coelenterata,  324-339. 
Coelome,  330. 
Coffee,  552. 
Colds,  528. 
Coleoptera,  389. 
Colon,  human,  470. 
Colony,    protozoan,    316;     hydroid, 

331 ;  Volvox,  318. 
Coloration,    protective,  of   frog,    27 ; 

of  insects,  399-403. 
Commensalism,  371. 
Composition,     chemical,     of     living 

matter,  8,  10-13. 
Compounds,  chemical,  7. 
Conifers,  214. 
Conjugation,    mold,    254 ;     Parame- 

cium,  303 ;   Spirogyra,  249. 
Conservation,  of  energy,  492-494. 
Control,  of  blood  flow,  491. 
Cooking,  effect  on  foods,  482. 
Coordination,     56,     509.     See     also 

nervous  activity. 
Copepod,  374. 
Coral-animals,  336-339. 
Cord,  spinal,  511. 
Corn,  grain,  149 ;   stalk,  168. 
Corpuscles,  blood,  483,  484. 
Cotyledons,  83;    146-150;    number 

of,  152 ;  work  of,  153. 
Cover  crops,  162. 
Crab,  370. 
Crayfish,  358-370. 
Crustaceans,  358-376. 
Cryptogams,  144,  233. 
Ctenophore,  335. 
Cultures,  bacteria,  280 ;   molds,  255- 

258 ;  yeast,  270. 
Cycle,  of  carbon,  126 ;    of  nitrogen, 

128;   of  organic  matter,  126. 
Cyst,  310. 

Cytoplasm,   see   protoplasm   of  cell- 
body. 


Daphnia,  374. 

Death,  animal,  17 ;   plant,  20. 


INDEX 


577 


Decapods,  369. 

Decomposition,  by  bacteria,  289. 

Dennis,  519. 

Development,  embryonic,  of  frog,  57- 
63;  of  plant,  198.  See  also 
embryology. 

Diaphragm,  human,  469,  504. 

Diastase,  105.  , 

Dicotyledon,  stem,  169-173. 

Diet,  529  ;   mixed,  501. 

Diffusion,  see  osmosis. 

Digestion,  animal,  46-49 ;  crayfish, 
366;  frog,  46-49;  human,  471- 
481 ;  Hydra,  326 ;  hygiene  of,  528- 
533 ;  plant,  105,  120. 

Dioecious,  243. 

Diptera,  389. 

Disease,  bacterial,  291,  554-560; 
human,  262 ;  and  insects,  393-398 ; 
of  plants,  259-261 ;  and  proto- 
zoans, 291,  312-315. 

Disinfection,  285,  558. 

Dispersal,  of  fruits  and  seeds,  223. 

Dissection,  of  animals,  25. 

Division,  Amceba,  310;  Parame- 
cium,  302.  See  also  cell-division. 

Dorsal,  26. 

Drugs,  553. 

Dust,  527. 

E 

Ear,  518. 

Earthworm,  349-353. 

Eating,  hygiene  of,  528. 

Echinoderms,  355-357. 

Economics,  of  seed-plants,  231  ; 
ferns,  241 ;  fungi,  246-263  ;  mush- 
rooms, 267 ;  algae,  251 ;  yeasts, 
275 ;  bacteria,  286-291 ;  proto- 
zoans, 312-316;  ccelenterates, 
339;  worms,  340-345,  350;  echi- 
noderms,  356;  decapods,  372; 
arachnids,  378;  insects,  390- 
398;  mollusks,  414-416;  fishes, 
422-424;  amphibians,  425,  426; 
reptiles,  427,  428 ;  mammals, 
441. 

Ectoderm,  322,  325. 

Egg-cell,  animal,  58;  plant,  79; 
fern,  236. 

2p 


Elements,  chemical,  6. 

Embryo,  animal,  59-62;  bean,  83; 
and  evolution,  564;  fish,  443; 
frog,  59-62;  fern,  238;  mammal, 
449-453;  plant,  198.  See  also 
reproduction. 

Embryology,  definitions,  17.  See 
embryo,  and  reproduction. 

Endoderm,  322,  325. 

Endosperm,  149. 

Energy,  124 ;  in  animals,  45 ;  conser- 
vation of,  492-494;  of  human 
body,  494  ;  in  foods,  495-497. 

Entomology,  380. 

Enzyme,  110,  129,  274.  See  also 
under  digestion. 

Epicotyl,  83,  153. 

Epidermis,  37 ;  plant,  174.  See  also 
skin. 

Epithelium,  37. 

Erosion,  and  roots,  161. 

Esophagus,  human,  469. 

Eustachian  tube,  human,  468,  519. 

Evaporation,  93,  95. 

Evolution,  561-571. 

Excess,  in  food,  in  exercise,  538. 

Excretion,  animal,  53 ;  animal  and 
plant  compared,  126 ;  frog,  53 ; 
human,  507;  Hydra,  328;  nitro- 
gen, 499;  plant,  113-115;  proto- 
zoans, 126 ;  worm,  352. 

Exercise,  537. 

Exoskeleton,  359. 

Eye,  human,  515. 


Fairy-rings,  265. 

Fats,  463. 

Feather,  431. 

Fehling's  reagent,  461. 

Fermentation,  268. 

Ferns,  233-241. 

Fertilization,  animal,  59 ;    plant,  79, 

197,  216,  236. 
Fertilizers,  for  soils,  99. 
Fibrin,  485. 
Fishes,  419-424. 
Flagella,  248,  322. 
Flea,  387,  389. 


578 


INDEX 


Flies,  397. 

Flowers,  76,  196-213;  functions  of, 
196. 

Flower-clock,  117. 

Flower-clusters,  208. 

Fluids,  digestive,  471. 

Foetus,  453. 

Foods,  of  animal,  14,  123  ;  energy  of, 
495-497;  human,  460-465;  need 
of,  44;  uses  of,  46;  of  plant,  19, 
98,  108,  119,  123;  of  protozoan, 
304,  309.  See  also  digestion,  and 
metabolism. 

Food-storage,  in  roots,  158 ;  in  stems, 
174 ;  in  leaves,  192. 

Food-supply,  human,  125. 

Forestry,  187. 

Frog,  14-17;  anatomy,  25-37;  his- 
tology, 37-44 ;  physiology,  44— 
57 ;  embryology,  57-64 ;  classifica- 
tion, 65. 

Fruits,  79-81,  216-225. 

Functions,  see  life-activities. 

Fungi,  245,  252-276;  food  of,  99- 
101. 

G 

Gametes,  238. 

Gametophyte,  239. 

Gametospore,  255. 

Ganglion,  352,  512. 

Gases,  from  organic  matter,  11. 
See  also  carbon  dioxide. 

Gasteropods,  411. 

Gastrula,  349. 

Gelatin  plates,  280. 

Generation,  spontaneous,  346. 

Generations,  alternation  of,  in  fern, 
238 ;  in  seed-plants,  239 ;  in  moss, 
244;  in  ccelenterates,  334,  335. 

Geology,  and  evolution,  565. 

Germ-cells,  in  heredity,  572.  See 
also  egg-cell,  sperm-cell. 

Germicides,  285. 

Germination,  84 ;   146-156. 

Germs,  135.     See  bacteria. 

Gestation,  450. 

Gills,  clam,  407;  crayfish,  361; 
embryos,  444;  fishes,  419;  tad- 
pole, 62. 


Gill-slits,  444. 
Girdling,  107. 
Glands,  salivary,  467 ;  liver  and 

pancreas,  470;  471. 
Grafting,  175,  227. 
Grains,  218. 
Grasshopper,  381,  392. 
Growth,  in  living  matter,  44 ;    foods 

for,  499;    secondary  in  stem,  169. 
Gullet,  see  esophagus. 
Gymnosperms,  213. 


H 


Hsematococcus,  249. 

Haemoglobin,  483. 

Hair,  521. 

Head,  of  flowers,  210. 

Heart,  487-490. 

Heat,  animal,  52  ;  effect  on  bacteria, 
283  ;  in  foods,  495,  498 ;  in  germi- 
nation, 155;  from  skin,  522-524. 

Hemiptera,  388. 

Heredity,  571-573. 

Hermaphroditism,  328. 

Hermit-crab,  370. 

Histology,  definition  of,  22 ;  of  frog, 
37-44. 

Horse,  fossil,  439. 

Horse  hair  worm,  345. 

Human  biology,  455. 

Hydra,  324-329. 

Hydranth,  332. 

Hydroids,  330-335. 

Hydrophobia,  296. 

Hygiene,  525-561. 

Hymenoptera,  389. 

Hyphas,  252,  264. 

Hypocotyl,  83,  154. 


Ichneumon-fly,  391. 

Imago,  383,  384. 

Immunity,  294. 

Incubation,  chick,  448. 

Inflorescence,  208. 

Infusoria,  319. 

Inheritance,  571. 

Inoculation,  256;    protective,  296. 


INDEX 


579 


Insects,     380-403;      in    pollination, 

199. 

Instincts,    birds,   434;    insects,   403. 
Inter-cellular  substance,  41. 
Interdependence     of     animals     and 

plants,   128. 
Intestines,  human,  470. 
Irritability,  129 ;  -Hydra,  328 ;  Para- 

mecium,    305;     plants,     116-119. 

See  also  nervous  activity. 


Jelly-fishes,  335. 


Kidneys,  frog,  35;   human,  508. 
King-crab,  378. 
Koch,  Dr.,  314. 


Labor,  division  of,  306,  329. 

Lanaellibranchs,  408. 

,  barnacle,  375 ;  butterfly, 
383;  frog,  63;  mosquito,  396, 
399 ;  worm,  349. 

Leaf,  73-76;  188,  196;  arrangement, 
189 ;  variegated,  103. 

Leaf-scars,  163. 

Lenticel,  112. 

Lepidoptera,  388. 

Lichens,  266. 

Life-activities,  animal,  13-18,  44- 
57  ;  in  cells,  41 ;  human,  457-523  ; 
plant,  18-20,  119-121. 

Light,  artificial,  on  plant  growth, 
104 ;  on  oxygen-supply  of  plants, 
111;  in  photosynthesis,  102,  111. 

Limulus,  378. 

Liquors,  see  alcohol. 

Liver-fluke,  343. 

Liver,  human,  470. 

Lobster,  358. 

Locomotion,  animal,  14 ;   plant,  18. 

Locusts,  382,  385. 

Lungs,  503. 

Lymph,  32,  485.  See  also  circula- 
tion. 


M 


Machinery,  of  life,  21. 

Maize,  149. 

Malaria,  312-314,  394-396. 

Mammals,  436-442. 

Man,  biology  and  classification,  455, 

456 ;    structure  and  life-activities, 

457-523. 

Manures,  and  bacteria,  287. 
Mastication,  471,  530. 
Matter,  organic,  2 ;    three  states  of, 

5;    gray,  512,  514;    white,  502. 
Medusa,  332,  335. 
Mesentery,  33. 
Metabolism,  53,  109,  120,  125,  499. 

See  also  growth,  repair,  assimilation. 
Metamere,  see  segment. 
Metamorphosis,     frog,     62 ;      insect, 

384 ;   worm,  349. 
Metazoa,  320. 
Microbes,  see  bacteria. 
Micro-organisms,   see   bacteria,    and 

protozoa. 

Migration,  bird,  434. 
Mildew,  259. 

Milk,  473  ;  bacteria  in,  288. 
Millipede,  379. 
Mimicry,  400. 
Minerals,  in  organic  matter,  12 ;    as 

foods,  464 ;   in  soil,  97. 
Molds,  252. 
Mollusca,  405-416. 
Molting,  bird,  431 ;    crustacean,  368. 
Monocotyledon,  152 ;  stem,  167. 
Monoecious,  243. 
Morphology,  definition  of,  21. 
Mosquitoes,  312,  393-396. 
Mosses,  241-245. 
Moth,  382. 

Mouth-cavity,  human,  465. 
Movement,  animal,  14 ;    blood,  486 ; 

plant,    18;    protozoan,    305,   308; 

respiratory,  504 ;   stomach,  472. 
Mulching,  87. 
Muscle,  cells,  Figs.  14,  15 ;    hygiene, 

537-539. 

Mushroom,  263-268. 
Mycelium,  252,  264. 
Myriopods,  379. 


580 


INDEX 


N 

Nails,  521. 

Names,  scientific,  133,  142. 

Narcotics,  539,  553. 

Nature-study,  of  frog,  24. 

Nautilus,  413. 

Nereis,  348. 

Nerve-endings,  510. 

Nerves,  511. 

Nervous  activity,  frog,  56,  57 ; 
human,  509-519;  worm,  352; 
reflex,  510;  voluntary,  511.  See 
also  irritability. 

Nervous  organs,  57,  509-519 ;  hy- 
giene, 535-537. 

Nettle-cells,  325. 

Neuroptera,  388. 

Nitrogen,  cycle,  128 ;  as  excretion, 
508 ;  in  foods,  499 ;  in  soils,  99. 

Nucleus,  40. 

Nutrients,  460. 

Nutrition,  animals,  56 ;  human, 
492-502. 

Nuts,  218. 

O 

Octopus,  413. 

Organisms,  2. 

Organs,  27  ;  of  frog,  36 ;  plant,  67. 

Orthoptera,  388. 

Osmosis,  89. 

Ovary,  in  flowers,  204. 

Overwork,  535. 

Oviparous  development,  63. 

Ovipositor,  381. 

Ovum,  see  egg-cell. 

Ovules,  78,  197. 

Oxidation,  7,  51,  125,  304. 

Oxygen,  in  animals,  52,  125;  in 
blood,  505 ;  in  cells,  506 ;  in  germi- 
nation, 155;  in  plants,  110,  119, 
125;  in  photosynthesis,  112. 

Oxygen-supply,  animal,  52,  55  ;  cray- 
fish, 366;  human,  502-506; 
Hydra,  327;  plant,  110-125. 

Oyster,  357,  409. 


Palate,  human,  465. 
Palisade  cells,  75. 


Pancreas,  human,  470. 
Paramecium,  300-307. 
Parasites,  barnacle,  375 ;  insect,  390  ; 

malarial,    312;     plant,    100,    259, 

267 ;  protozoan,   312-315 ;  worms, 

341-345. 

Parthenogenesis,  196,  386. 
Pasteur,  269,  284. 
Pasteurization,  284. 
Pepsin,  477. 
Perianth,  203. 
Pericardium,  30,  487. 
Perennial,  177. 
Petal,  203. 
Petri  dish,  257. 
Phanerogams,  233. 
Pharynx,  human,  468. 
Photosynthesis,  102. 
Phylum,  137. 
Physics,  definition  of,  6. 
Physiology,      22 ;      animal,     44-57 ; 

human,  457-524;    plant,    85-120. 

See  also  under  descriptions  of  types 

of  animals  and  plants  in  Parts  II 

and  III. 
Pistil,  205. 
Pith-rays,  170. 

Placenta,  animal,  452  ;   plant,  80. 
Planaria,  341. 
Plant,  biology  of,  66-121. 
Plant-lice,  386. 
Plants,  group  of,  144. 
Pleura,  503. 
Pleurisy,  503. 
Pleurococcus,  237. 
Plumule,  83. 
Pod,  79-81. 
Pollen-grain,  197,  216. 
Pollination,    77,    78,    197,    200,    201, 

216,  390. 
Polyp,  332. 
Porifera,  320-324. 
Posterior,  26. 
Pregnancy,  450. 
Propagation,  from  roots,   162 ;  from 

stems,    180;    of  seed-plants,   225- 

227.     See  also  reproduction. 
Proteins,  104,  497-500,  530. 
Prothallium,  235. 
Protonema,  243. 


INDEX 


581 


Protoplasm,  43,  69,  71,  122,  499. 

Protozoa,  300-319. 

Pruning,  176. 

Pseudopod,  309. 

Psychology,  of  digestion,  532. 

Pteridophytes,  240. 

Ptyalin,  476. 

Puff-ball,  268. 

Pulse,  490. 

Pupa,  383,  384. 

Pylorus,  33. 

Q 

Quarter-sawing,  186. 


Radicle,  83. 

Reflexes,  510. 

Regeneration,  of  Hydra,  329 ;  of 
starfish,  357. 

Rennin,  477. 

Repair,  44,  499. 

Reproduction,  16,  27 ;  asexual  plant, 
225-227 ;  asexual  animal  (see 
budding)  ;  frog,  57-65 ;  bean,  76- 
85 ;  animal  and  plant  compared, 
131;  seed-plants,  146-156,  196- 
227;  fern,  235-240;  moss,  243- 
245 ;  various  spore-plants,  246- 
254 ;  Paramecium,  302,  305  ;  amoe- 
ba, 310;  sponge,  322;  Hydra, 
328;  hydroids,  334;  corals,  337; 
earthworm,  351-353 ;  insects, 
384-387  ;  vertebrates,  442-453  ; 
fishes,  443;  birds,  446;  reptiles, 
447 ;  mammals,  447-453. 

Reptiles,  426-428. 

Resemblancest  -jn  classification,   138. 

Respiration,  55;  human,  502-508; 
plant,  110.  See  also  oxygen-sup- 
ply, and  excretion  of  CO2. 

Retina,  516. 

Rhizoids,  236. 

Rings,  annual,  170. 

Root-cap,  70. 

Root-hairs,  69,  89. 

Root-pressure,  89. 

Roots,  68-70;  88;  156-163. 


Rootstock,  182  ;   fern,  233. 
Root-tubercles,  69,  159,  287. 
Rudiments,  563. 
Rusts,  259. 


Saliva,  human,  467. 

Salmon,  422. 

Sandworm,  348. 

Sanitation,  526. 

Sap,  130. 

Saprophyte,  100. 

Scale-leaves,  193. 

Sciences,  2. 

Scorpion,  377. 

Sea-anemone,  336. 

Sea-fans,  338. 

Sea-urchin,  355. 

Sea-weeds,  246,  251. 

Secretions,  48;  digestive,  474;  sali- 
vary, 474  ;  gastrict.477 ;  intestinal, 
479  ;  pancreatic,  480 ;  bile,  480. 

Seed-distributionr^lO. 

Seedlings,  146-156. 

Seed-plants,  146-231. 

Seeds,  bean,  81-83;  various,  146- 
156. 

Segmentxcrayfish,  360 ;  insects,  381  ; 
worm,  347. 

Sense-organs,  27,  362. 

Senses,  special,  515-519. 

Sensitive  plant,  18. 

Sepal,  203. 

Sewage,  bacteria  in,  289. 

Shad,  423. 

Sieve-tubes,  work  of,  106. 

Silver-moth,  387. 

Skeleton,  coral,  337 ;  crustacean, 
359;  frog,  30;  human,  457; 
insect,  381. 

Skin,  human,  508,  519,  521-524; 
hygiene,  533-535. 

Sleep,  of  plants,  118;   human,  536. 

Sleeping  sickness,  314. 

Slugs,  411. 

Smallpox,  295. 

Smut,  259. 

Snail,  410. 

Soap,  in  hygiene,  533. 


582 


INDEX 


Soils,  and  bacteria,  286 ;  plant  food- 
materials  in,  99 ;  and  worms,  350 ; 
water  in,  87. 

Solutions,  47. 

Sow-bug,  373. 

Species,  135. 

Sperm  aphyta,  see  seed-plants. 

Sperm-cell,  58;  plant,  197;  fern, 
236. 

Sphffirella,  247. 

Spider,  376. 

Spiracle,  381. 

Spirogyra,  249. 

Sponge-animals,  320-324. 

Sporangia,  see  spore-case. 

Spore-case,  215,  235,  253. 

Spore-plants,  232-298. 

Spores,  232,  235,  243,  279,  283,  312. 

Sporophyll,  215,  235. 

Sporophyte,  239, 

Squid,  413. 

Stamens,  207. 

Starch,  in  leaves,  104 ;  test  for,  102 ; 
formula,  111;  as  food,  462. 

Starfish,  355. 

Stem,  70-72;  163-188;  functions, 
179 ;  as  leaves,  184 ;  adaptations, 
191. 

Sterilization,  256,  284. 

Sterilizer,  256. 

Stimulants,  531,  53  -553. 

Stimuli,  effect  on  plants,  116-119. 

Stipules,  193. 

Stoma,  74,  75 ;  work  of,  96. 

Structure,  see  descriptions  of  various 
animals  and  plants. 

Struggle  for  existence,  568. 

Sugar,  test  for,  105 ;  as  food,  461, 
462. 

Swallowing,  471,  472. 

Sweat-glands,  521,  522. 

Symbiosis,  329. 

Symmetry,  bilateral,  and  radial,  26. 

Systems,  of  organs,  27,  36. 


Tadpole,  62. 
Tape- worm,  340. 
Taste,  519. 


Tea,  552. 

Teeth,  466 ;   hygiene  of,  528. 

Temperature,  sense,  519. 

Tendrils,  180. 

Tentacle,  325. 

Tests,  for  foods,  460-462. 

Thallophytes,  246. 

Thorns,  181,  193. 

Tissues,    animal,    37-43 ;    plant,   69, 

71,  72,  74,  123. 
Toad,  24. 
Tobacco,  551. 
Tongue,  human,  467. 
Tonsil,  468. 
Touch,  519. 
Toxins,  293. 
Trachea?,    human,    502 ;     of   insects, 

381. 

Transpiration,  97. 
Trichina,  344. 
Tropisms,  118. 
Tubers,  182. 
Twigs,  163. 
Tyndall,  347. 


U 


Unicellular  animals,  300-307. 
Univalves,  411. 
Ureter,  508. 

Use  of  animals  and  plants  (see  eco- 
nomics) ;  in  science,  24. 
Uterus,  450. 


Vaccination,  295. 

Yacuole,  contractile,  302,  309 ;   food, 

302 ;   yeast,  269. 
Valves,  heart,  487  ;  veins,  489. 
Varieties,  dog,  138;   beans,  81. 
Veining,  of  leaves,  194. 
Veins,  31,  489. 
Ventilation,  527. 
Ventral,  26. 
Venus  fly-trap,  18. 
Vermes,  340. 
Vertebra,  457. 
Vertebrates,  417-453. 
Vinegar,  276,  289. 


INDEX 


583 


Vinegar-eel,  344. 
Viviparous  development,  63. 
Vivisection,  25. 
Volvox,  318. 
Vorticella,  317. 

,W 

Waste,  in  living  matter,  44.  See  also 
excretions. 

Water,  8;  as  excretion,  114,  509;  in 
germination,  154;  hygiene,  529; 
in  leaf,  95 ;  need  of,  95 ;  in  organ- 
isms, 10,  44 ;  in  roots,  92  ;  in  soil, 
87  ;  in  stems,  94 ;  use  of  in  plant, 
97 ;  in  transportation,  130. 


Water-flea,  374. 
Wood,  184. 
Wood-lice,  373. 
Work,  see  energy. 
Worms,  340-354. 


Yeast,  268. 
Yellow  fever,  394. 
Yolk,  447. 

Z 

Zoology,  definition  of,  2. 
Zygospore,  255. 


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